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`.

models/migrations/v1_24/v312.go

@@ -0,0 +1,21 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_24 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+type pullAutoMerge struct {
+	DeleteBranchAfterMerge bool
+}
+
+// TableName return database table name for xorm
+func (pullAutoMerge) TableName() string {
+	return "pull_auto_merge"
+}
+
+func AddDeleteBranchAfterMergeForAutoMerge(x *xorm.Engine) error {
+	return x.Sync(new(pullAutoMerge))
+}