Backport #25330

# The problem

There were many "path tricks":

* By default, Gitea uses its program directory as its work path
* Gitea tries to use the "work path" to guess its "custom path" and
"custom conf (app.ini)"
* Users might want to use other directories as work path
* The non-default work path should be passed to Gitea by GITEA_WORK_DIR
or "--work-path"
* But some Gitea processes are started without these values
    * The "serv" process started by OpenSSH server
    * The CLI sub-commands started by site admin
* The paths are guessed by SetCustomPathAndConf again and again
* The default values of "work path / custom path / custom conf" can be
changed when compiling

# The solution

* Use `InitWorkPathAndCommonConfig` to handle these path tricks, and use
test code to cover its behaviors.
* When Gitea's web server runs, write the WORK_PATH to "app.ini", this
value must be the most correct one, because if this value is not right,
users would find that the web UI doesn't work and then they should be
able to fix it.
* Then all other sub-commands can use the WORK_PATH in app.ini to
initialize their paths.
* By the way, when Gitea starts for git protocol, it shouldn't output
any log, otherwise the git protocol gets broken and client blocks
forever.

The "work path" priority is: WORK_PATH in app.ini > cmd arg --work-path
> env var GITEA_WORK_DIR > builtin default

The "app.ini" searching order is: cmd arg --config > cmd arg "work path
/ custom path" > env var "work path / custom path" > builtin default


## ⚠️ BREAKING

If your instance's "work path / custom path / custom conf" doesn't meet
the requirements (eg: work path must be absolute), Gitea will report a
fatal error and exit. You need to set these values according to the
error log.

This PR will merge 3 Init functions on setting packages as 1 and
introduce an options struct.

# Why this PR comes

At first, I'd like to help users like #23636 (there are a lot)

The unclear "Internal Server Error" is quite anonying, scare users,
frustrate contributors, nobody knows what happens.

So, it's always good to provide meaningful messages to end users (of
course, do not leak sensitive information).

When I started working on the "response message to end users", I found
that the related code has a lot of technical debt. A lot of copy&paste
code, unclear fields and usages.

So I think it's good to make everything clear.

# Tech Backgrounds

Gitea has many sub-commands, some are used by admins, some are used by
SSH servers or Git Hooks. Many sub-commands use "internal API" to
communicate with Gitea web server.

Before, Gitea server always use `StatusCode + Json "err" field` to
return messages.

* The CLI sub-commands: they expect to show all error related messages
to site admin
* The Serv/Hook sub-commands (for git clients): they could only show
safe messages to end users, the error log could only be recorded by
"SSHLog" to Gitea web server.

In the old design, it assumes that:

* If the StatusCode is 500 (in some functions), then the "err" field is
error log, shouldn't be exposed to git client.
* If the StatusCode is 40x, then the "err" field could be exposed. And
some functions always read the "err" no matter what the StatusCode is.

The old code is not strict, and it's difficult to distinguish the
messages clearly and then output them correctly.

# This PR

To help to remove duplicate code and make everything clear, this PR
introduces `ResponseExtra` and `requestJSONResp`.

* `ResponseExtra` is a struct which contains "extra" information of a
internal API response, including StatusCode, UserMsg, Error
* `requestJSONResp` is a generic function which can be used for all
cases to help to simplify the calls.
* Remove all `map["err"]`, always use `private.Response{Err}` to
construct error messages.
* User messages and error messages are separated clearly, the `fail` and
`handleCliResponseExtra` will output correct messages.
* Replace all `Internal Server Error` messages with meaningful (still
safe) messages.

This PR saves more than 300 lines, while makes the git client messages
more clear.

Many gitea-serv/git-hook related essential functions are covered by
tests.

---------

Co-authored-by: delvh <dev.lh@web.de>

Some bugs caused by less unit tests in fundamental packages. This PR
refactor `setting` package so that create a unit test will be easier
than before.

- All `LoadFromXXX` files has been splited as two functions, one is
`InitProviderFromXXX` and `LoadCommonSettings`. The first functions will
only include the code to create or new a ini file. The second function
will load common settings.
- It also renames all functions in setting from `newXXXService` to
`loadXXXSetting` or `loadXXXFrom` to make the function name less
confusing.
- Move `XORMLog` to `SQLLog` because it's a better name for that.

Maybe we should finally move these `loadXXXSetting` into the `XXXInit`
function? Any idea?

---------

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: delvh <dev.lh@web.de>

Change all license headers to comply with REUSE specification.

Fix #16132

Co-authored-by: flynnnnnnnnnn <flynnnnnnnnnn@github>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

* Improve install code to avoid low-level mistakes.

If a user tries to do a re-install in a Gitea database, they gets a warning and double check.
When Gitea runs, it never create empty app.ini automatically.

Also some small (related) refactoring:

* Refactor db.InitEngine related logic make it more clean (especially for the install code)
* Move some i18n strings out from setting.go to make the setting.go can be easily maintained.
* Show errors in CLI code if an incorrect app.ini is used.
* APP_DATA_PATH is created when installing, and checked when starting (no empty directory is created any more).

* go build / format tools
* re-format imports

* Second attempt at preventing zombies

* Ensure that the pipes are closed in ssh.go
* Ensure that a cancellable context is passed up in cmd/* http requests
* Make cmd.fail return properly so defers are obeyed
* Ensure that something is sent to stdout in case of blocks here

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint 2

Signed-off-by: Andrew Thornton <art27@cantab.net>

* placate lint 3

Signed-off-by: Andrew Thornton <art27@cantab.net>

* fixup

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Apply suggestions from code review

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lauris BH <lauris@nix.lv>

* Fix send mail

* Fix send mail

* Update modules/private/mail.go

Co-authored-by: techknowlogick <matti@mdranta.net>

* Add SendSync method

Usefull to have when you need to be confident that message was sent.

* Add sendmail command

* add checks that if either title or content is empty then error out

* Add a confirmation step

* Add --force option to bypass confirm step

* Move implementation of runSendMail to a different file

* Add copyrighting comment

* Make content optional

Print waring if it's empty or haven't been set up.
The warning will be skiped if there's a `--force` flag.

* Fix import style

Co-authored-by: 6543 <6543@obermui.de>

* Use batch when getting all users

IterateUsers uses batching by default.

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Send emails one by one instead of as one chunck

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Send messages concurantly

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Use SendAsync+Flush instead of SendSync

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Add timeout parameter to sendemail command

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Fix spelling mistake

Signed-off-by: Maxim Zhiburt <zhiburt@gmail.com>

* Update cmd/admin.go

Co-authored-by: 6543 <6543@obermui.de>

* Connect to a running Gitea instance

* Fix mispelling

* Add copyright comment

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>