Just like `models/unittest`, the testing helper functions should be in a
separate package: `contexttest`

And complete the TODO:

> // TODO: move this function to other packages, because it depends on
"models" package

The web context (modules/context.Context) is quite complex, it's
difficult for the callers to initialize correctly.

This PR introduces a `NewWebContext` function, to make sure the web
context have the same behavior for different cases.

Replace #16455

Close #21803

Mixing different Gitea contexts together causes some problems:

1. Unable to respond proper content when error occurs, eg: Web should
respond HTML while API should respond JSON
2. Unclear dependency, eg: it's unclear when Context is used in
APIContext, which fields should be initialized, which methods are
necessary.


To make things clear, this PR introduces a Base context, it only
provides basic Req/Resp/Data features.

This PR mainly moves code. There are still many legacy problems and
TODOs in code, leave unrelated changes to future PRs.

Support RTL content in Markdown:


![image](https://github.com/go-gitea/gitea/assets/115237/dedb1b0c-2f05-40dc-931a-0d9dc81f7c97)

Example document:
https://try.gitea.io/silverwind/symlink-test/src/branch/master/bidi-text.md
Same on GitHub:
https://github.com/silverwind/symlink-test/blob/master/bidi-text.md

`dir=auto` enables a browser heuristic that sets the text direction
automatically. It is the only way to get automatic text direction.

Ref: https://codeberg.org/Codeberg/Community/issues/1021

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

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>

We need to make sure a user can't confirm the existence of a user with
private visibility

* Follow up on #21533 

### Before
#### User

![image](https://user-images.githubusercontent.com/20454870/197357580-340911d7-1659-4fc9-a9f6-7ed6bc3476b4.png)

#### Admin

![image](https://user-images.githubusercontent.com/20454870/197357676-a8f0ae63-8f80-4221-a9b5-b6311552910a.png)

### After
#### User

![image](https://user-images.githubusercontent.com/20454870/197357536-05616edb-7821-469d-8e51-6f8cb84c1362.png)

#### Admin

![image](https://user-images.githubusercontent.com/20454870/197357703-071fe984-de79-43aa-a77c-a85b046292a4.png)

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Previously mentioning a user would link to its profile, regardless of
whether the user existed. This change tests if the user exists and only
if it does - a link to its profile is added.

* Fixes #3444

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>