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/web/repo/branch.go

@@ -97,7 +97,7 @@ func DeleteBranchPost(ctx *context.Context) {
 	defer redirect(ctx)
 	branchName := ctx.FormString("name")
 
-	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):
 			log.Debug("DeleteBranch: Can't delete non existing branch '%s'", branchName)