Automerge supports deleting branch automatically after merging (#32343)

Resolve #32341 
~Depends on #27151~

- [x] It will display a checkbox of deleting the head branch on the pull
request view page when starting an auto-merge task.
- [x] Add permission check before deleting the branch
- [x] Add delete branch comment for those closing pull requests because
of head branch or base branch was deleted.
- [x] Merge `RetargetChildrenOnMerge` and `AddDeletePRBranchComment`
into `service.DeleteBranch`.

routers/api/v1/repo/branch.go

@@ -150,7 +150,7 @@ func DeleteBranch(ctx *context.APIContext) {
 		}
 	}
 
-	if err := repo_service.DeleteBranch(ctx, ctx.Doer, ctx.Repo.Repository, ctx.Repo.GitRepo, branchName); err != nil {
+	if err := repo_service.DeleteBranch(ctx, ctx.Doer, ctx.Repo.Repository, ctx.Repo.GitRepo, branchName, nil); err != nil {
 		switch {
 		case git.IsErrBranchNotExist(err):
 			ctx.NotFound(err)