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/repo/issue.go

@@ -388,6 +388,14 @@ func issues(ctx *context.Context, milestoneID, projectID int64, isPullOption uti
 		return
 	}
 
+	pinned, err := issues_model.GetPinnedIssues(ctx, repo.ID, isPullOption.IsTrue())
+	if err != nil {
+		ctx.ServerError("GetPinnedIssues", err)
+		return
+	}
+
+	ctx.Data["PinnedIssues"] = pinned
+	ctx.Data["IsRepoAdmin"] = ctx.IsSigned && (ctx.Repo.IsAdmin() || ctx.Doer.IsAdmin)
 	ctx.Data["IssueStats"] = issueStats
 	ctx.Data["SelLabelIDs"] = labelIDs
 	ctx.Data["SelectLabels"] = selectLabels
@@ -1854,6 +1862,17 @@ func ViewIssue(ctx *context.Context) {
 		return
 	}
 
+	var pinAllowed bool
+	if !issue.IsPinned() {
+		pinAllowed, err = issues_model.IsNewPinAllowed(ctx, issue.RepoID, issue.IsPull)
+		if err != nil {
+			ctx.ServerError("IsNewPinAllowed", err)
+			return
+		}
+	} else {
+		pinAllowed = true
+	}
+
 	ctx.Data["Participants"] = participants
 	ctx.Data["NumParticipants"] = len(participants)
 	ctx.Data["Issue"] = issue
@@ -1865,6 +1884,8 @@ func ViewIssue(ctx *context.Context) {
 	ctx.Data["IsRepoAdmin"] = ctx.IsSigned && (ctx.Repo.IsAdmin() || ctx.Doer.IsAdmin)
 	ctx.Data["LockReasons"] = setting.Repository.Issue.LockReasons
 	ctx.Data["RefEndName"] = git.RefEndName(issue.Ref)
+	ctx.Data["NewPinAllowed"] = pinAllowed
+	ctx.Data["PinEnabled"] = setting.Repository.Issue.MaxPinned != 0
 
 	var hiddenCommentTypes *big.Int
 	if ctx.IsSigned {