Add "Allow edits from maintainer" feature (#18002)

Adds a feature [like GitHub has](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request-from-a-fork) (step 7).
If you create a new PR from a forked repo, you can select (and change later, but only if you are the PR creator/poster) the "Allow edits from maintainers" option.
Then users with write access to the base branch get more permissions on this branch:
* use the update pull request button
* push directly from the command line (`git push`)
* edit/delete/upload files via web UI
* use related API endpoints

You can't merge PRs to this branch with this enabled, you'll need "full" code write permissions.

This feature has a pretty big impact on the permission system. I might forgot changing some things or didn't find security vulnerabilities. In this case, please leave a review or comment on this PR.

Closes #17728

Co-authored-by: 6543 <6543@obermui.de>

routers/api/v1/repo/pull.go

@@ -616,6 +616,18 @@ func EditPullRequest(ctx *context.APIContext) {
 		notification.NotifyPullRequestChangeTargetBranch(ctx.Doer, pr, form.Base)
 	}
 
+	// update allow edits
+	if form.AllowMaintainerEdit != nil {
+		if err := pull_service.SetAllowEdits(ctx, ctx.Doer, pr, *form.AllowMaintainerEdit); err != nil {
+			if errors.Is(pull_service.ErrUserHasNoPermissionForAction, err) {
+				ctx.Error(http.StatusForbidden, "SetAllowEdits", fmt.Sprintf("SetAllowEdits: %s", err))
+				return
+			}
+			ctx.ServerError("SetAllowEdits", err)
+			return
+		}
+	}
+
 	// Refetch from database
 	pr, err = models.GetPullRequestByIndex(ctx.Repo.Repository.ID, pr.Index)
 	if err != nil {