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`.

tests/integration/actions_trigger_test.go

@@ -405,7 +405,7 @@ func TestCreateDeleteRefEvent(t *testing.T) {
 		assert.NotNil(t, run)
 
 		// delete the branch
-		err = repo_service.DeleteBranch(db.DefaultContext, user2, repo, gitRepo, "test-create-branch")
+		err = repo_service.DeleteBranch(db.DefaultContext, user2, repo, gitRepo, "test-create-branch", nil)
 		assert.NoError(t, err)
 		run = unittest.AssertExistsAndLoadBean(t, &actions_model.ActionRun{
 			Title:      "add workflow",