The xorm `Sync2` has already been deprecated in favor of `Sync`,
so let's do the same inside the Gitea codebase.

Command used to replace everything:
```sh
for i in $(ag Sync2 --files-with-matches); do vim $i -c ':%sno/Sync2/Sync/g' -c ':wq'; done
```

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>

There are too many files in `models/migrations` folder so that I split
them into sub folders.

* Create Proper Migration tests

Unfortunately our testing regime has so far meant that migrations do not
get proper testing.

This PR begins the process of creating migration tests for this.

* Add test for v176

* fix mssql drop db

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

* Fix Migration 176 yet again

Whilst creating a test for v176 in the migrations_test PR
it has become clear that this was still wrong.

This is now fixed. Genuinely.

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

* and fix repo transfer

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

There is a serious issue with the v176 migration where there is a mistaken missing
label_id selection.

*introduced by #14912*

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

* Never add labels not from this repository or organisation and remove org labels on transfer

Prevent the addition of labels from outside of the repository or
organisation and remove organisation labels on transfer.

Related #14908

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

* switch to use sql

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

* remove AS

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

* subquery alias

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

* Give me some AS?

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

* double AS

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

* try try again

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

* once more around the merry go round

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

* fix api problem

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

* Add outside label consistency check into doctor

This PR adds another consistency check into doctor in order to detect
labels that have been added from outside of repositories and organisations

Fix #14908

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

* fix migration

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

* prep for merge

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

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