On creation of an empty project (no template) a default board will be
created instead of falling back to the uneditable pseudo-board.

Every project now has to have exactly one default boards. As a
consequence, you cannot unset a board as default, instead you have to
set another board as default. Existing projects will be modified using a
cron job, additionally this check will run every midnight by default.

Deleting the default board is not allowed, you have to set another board
as default to do it.

Fixes #29873
Fixes #14679 along the way
Fixes #29853

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

Follow #28662

Part of #27065

This PR touches functions used in templates. As templates are not static
typed, errors are harder to find, but I hope I catch it all. I think
some tests from other persons do not hurt.

Change 
`GetBoards(ctx context.Context, projectID int64)`
to
`(p *Project) GetBoards(ctx context.Context) (BoardList, error)`

`getDefaultBoard` is same.

Original Issue: https://github.com/go-gitea/gitea/issues/22102

This addition would be a big benefit for design and art teams using the
issue tracking.

The preview will be the latest "image type" attachments on an issue-
simple, and allows for automatic updates of the cover image as issue
progress is made!

This would make Gitea competitive with Trello... wouldn't it be amazing
to say goodbye to Atlassian products? Ha.

First image is the most recent, the SQL will fetch up to 5 latest images
(URL string).

All images supported by browsers plus upcoming formats: *.avif *.bmp
*.gif *.jpg *.jpeg *.jxl *.png *.svg *.webp

The CSS will try to center-align images until it cannot, then it will
left align with overflow hidden. Single images get to be slightly
larger!

Tested so far on: Chrome, Firefox, Android Chrome, Android Firefox.

Current revision with light and dark themes:

![image](https://user-images.githubusercontent.com/24665/207066878-58e6bf73-0c93-4caa-8d40-38f4432b3578.png)


![image](https://user-images.githubusercontent.com/24665/207066555-293f65c3-e706-4888-8516-de8ec632d638.png)

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
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>

Fix #19513

This PR introduce a new db method `InTransaction(context.Context)`,
and also builtin check on `db.TxContext` and `db.WithTx`.
There is also a new method `db.AutoTx` has been introduced but could be used by other PRs.

`WithTx` will always open a new transaction, if a transaction exist in context, return an error.
`AutoTx` will try to open a new transaction if no transaction exist in context.
That means it will always enter a transaction if there is no error.

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

* Move almost all functions' parameter db.Engine to context.Context
* remove some unnecessary wrap functions

* Move project files into models/project sub package

* Fix test

* Fix test

* Fix test

* Fix build

* Fix test

* Fix template bug

* Fix bug

* Fix lint

* Fix test

* Fix import

* Improve codes

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