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>

templates/repo/issue/view_content/comments.tmpl

@@ -11,7 +11,7 @@
 		26 = DELETE_TIME_MANUAL, 27 = REVIEW_REQUEST, 28 = MERGE_PULL_REQUEST,
 		29 = PULL_PUSH_EVENT, 30 = PROJECT_CHANGED, 31 = PROJECT_BOARD_CHANGED
 		32 = DISMISSED_REVIEW, 33 = COMMENT_TYPE_CHANGE_ISSUE_REF, 34 = PR_SCHEDULE_TO_AUTO_MERGE,
-		35 = CANCEL_SCHEDULED_AUTO_MERGE_PR -->
+		35 = CANCEL_SCHEDULED_AUTO_MERGE_PR, 36 = PIN_ISSUE, 37 = UNPIN_ISSUE -->
 		{{if eq .Type 0}}
 			<div class="timeline-item comment" id="{{.HashTag}}">
 			{{if .OriginalAuthor}}
@@ -835,6 +835,16 @@
 					{{else}}{{$.locale.Tr "repo.pulls.auto_merge_canceled_schedule_comment" $createdStr | Safe}}{{end}}
 				</span>
 			</div>
+		{{else if or (eq .Type 36) (eq .Type 37)}}
+			<div class="timeline-item event" id="{{.HashTag}}">
+				<span class="badge">{{svg "octicon-pin" 16}}</span>
+				{{template "shared/user/avatarlink" dict "Context" $.Context "user" .Poster}}
+				<span class="text grey muted-links">
+					{{template "shared/user/authorlink" .Poster}}
+					{{if eq .Type 36}}{{$.locale.Tr "repo.issues.pin_comment" $createdStr | Safe}}
+					{{else}}{{$.locale.Tr "repo.issues.unpin_comment" $createdStr | Safe}}{{end}}
+				</span>
+			</div>
 		{{end}}
 	{{end}}
 {{end}}