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>

routers/web/web.go

@@ -987,6 +987,7 @@ func registerRoutes(m *web.Route) {
 				m.Post("/deadline", web.Bind(structs.EditDeadlineOption{}), repo.UpdateIssueDeadline)
 				m.Post("/watch", repo.IssueWatch)
 				m.Post("/ref", repo.UpdateIssueRef)
+				m.Post("/pin", reqRepoAdmin, repo.IssuePinOrUnpin)
 				m.Post("/viewed-files", repo.UpdateViewedFiles)
 				m.Group("/dependency", func() {
 					m.Post("/add", repo.AddDependency)
@@ -1024,6 +1025,8 @@ func registerRoutes(m *web.Route) {
 			m.Post("/resolve_conversation", reqRepoIssuesOrPullsReader, repo.UpdateResolveConversation)
 			m.Post("/attachments", repo.UploadIssueAttachment)
 			m.Post("/attachments/remove", repo.DeleteAttachment)
+			m.Delete("/unpin/{id}", reqRepoAdmin, repo.IssueUnpin)
+			m.Post("/pin_move", reqRepoAdmin, repo.IssuePinMove)
 		}, context.RepoMustNotBeArchived())
 		m.Group("/comments/{id}", func() {
 			m.Post("", repo.UpdateCommentContent)