Add the ability to pin Issues (#24406)

This adds the ability to pin important Issues and Pull Requests. You can
also move pinned Issues around to change their Position. Resolves #2175.

## Screenshots

![grafik](https://user-images.githubusercontent.com/15185051/235123207-0aa39869-bb48-45c3-abe2-ba1e836046ec.png)

![grafik](https://user-images.githubusercontent.com/15185051/235123297-152a16ea-a857-451d-9a42-61f2cd54dd75.png)

![grafik](https://user-images.githubusercontent.com/15185051/235640782-cbfe25ec-6254-479a-a3de-133e585d7a2d.png)

The Design was mostly copied from the Projects Board.

## Implementation
This uses a new `pin_order` Column in the `issue` table. If the value is
set to 0, the Issue is not pinned. If it's set to a bigger value, the
value is the Position. 1 means it's the first pinned Issue, 2 means it's
the second one etc. This is dived into Issues and Pull requests for each
Repo.

## TODO
- [x] You can currently pin as many Issues as you want. Maybe we should
add a Limit, which is configurable. GitHub uses 3, but I prefer 6, as
this is better for bigger Projects, but I'm open for suggestions.
- [x] Pin and Unpin events need to be added to the Issue history.
- [x] Tests
- [x] Migration

**The feature itself is currently fully working, so tester who may find
weird edge cases are very welcome!**

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

custom/conf/app.example.ini

@@ -1048,6 +1048,9 @@ LEVEL = Info
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; List of reasons why a Pull Request or Issue can be locked
 ;LOCK_REASONS = Too heated,Off-topic,Resolved,Spam
+;; Maximum number of pinned Issues
+;; Set to 0 to disable pinning Issues
+;MAX_PINNED = 3
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;