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_pin.go

@@ -0,0 +1,88 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"net/http"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/modules/context"
+	"code.gitea.io/gitea/modules/json"
+)
+
+// IssuePinOrUnpin pin or unpin a Issue
+func IssuePinOrUnpin(ctx *context.Context) {
+	issue := GetActionIssue(ctx)
+
+	// If we don't do this, it will crash when trying to add the pin event to the comment history
+	err := issue.LoadRepo(ctx)
+	if err != nil {
+		ctx.Status(http.StatusInternalServerError)
+		return
+	}
+
+	err = issue.PinOrUnpin(ctx, ctx.Doer)
+	if err != nil {
+		ctx.Status(http.StatusInternalServerError)
+		return
+	}
+
+	ctx.Redirect(issue.Link())
+}
+
+// IssueUnpin unpins a Issue
+func IssueUnpin(ctx *context.Context) {
+	issue, err := issues_model.GetIssueByIndex(ctx.Repo.Repository.ID, ctx.ParamsInt64(":id"))
+	if err != nil {
+		ctx.Status(http.StatusNoContent)
+		return
+	}
+
+	// If we don't do this, it will crash when trying to add the pin event to the comment history
+	err = issue.LoadRepo(ctx)
+	if err != nil {
+		ctx.Status(http.StatusInternalServerError)
+		return
+	}
+
+	err = issue.Unpin(ctx, ctx.Doer)
+	if err != nil {
+		ctx.Status(http.StatusInternalServerError)
+	}
+
+	ctx.Status(http.StatusNoContent)
+}
+
+// IssuePinMove moves a pinned Issue
+func IssuePinMove(ctx *context.Context) {
+	if ctx.Doer == nil {
+		ctx.JSON(http.StatusForbidden, "Only signed in users are allowed to perform this action.")
+		return
+	}
+
+	type movePinIssueForm struct {
+		ID       int64 `json:"id"`
+		Position int   `json:"position"`
+	}
+
+	form := &movePinIssueForm{}
+	if err := json.NewDecoder(ctx.Req.Body).Decode(&form); err != nil {
+		ctx.Status(http.StatusInternalServerError)
+		return
+	}
+
+	issue, err := issues_model.GetIssueByID(ctx, form.ID)
+	if err != nil {
+		ctx.Status(http.StatusInternalServerError)
+		return
+	}
+
+	err = issue.MovePin(ctx, form.Position)
+	if err != nil {
+		ctx.Status(http.StatusInternalServerError)
+		return
+	}
+
+	ctx.Status(http.StatusNoContent)
+}