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>

options/locale/locale_en-US.ini

@@ -115,6 +115,9 @@ unknown = Unknown
 
 rss_feed = RSS Feed
 
+pin = Pin
+unpin = Unpin
+
 artifacts = Artifacts
 
 concept_system_global = Global
@@ -1482,6 +1485,10 @@ issues.attachment.open_tab = `Click to see "%s" in a new tab`
 issues.attachment.download = `Click to download "%s"`
 issues.subscribe = Subscribe
 issues.unsubscribe = Unsubscribe
+issues.unpin_issue = Unpin Issue
+issues.max_pinned = "You can't pin more issues"
+issues.pin_comment = "pinned this %s"
+issues.unpin_comment = "unpinned this %s"
 issues.lock = Lock conversation
 issues.unlock = Unlock conversation
 issues.lock.unknown_reason = Cannot lock an issue with an unknown reason.