Refactor update checker to use AppState (#17387)

We have the `AppState` module now, it can store app related data easily. We do not need to create separate tables for each feature.

So the update checker can use `AppState` instead of a one-row dedicate table.

And the code of update checker is moved from `models` to `modules`.

models/migrations/v201.go

@@ -0,0 +1,15 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package migrations
+
+import (
+	"xorm.io/xorm"
+)
+
+func dropTableRemoteVersion(x *xorm.Engine) error {
+	// drop the orphaned table introduced in `v199`, now the update checker also uses AppState, do not need this table
+	_ = x.DropTables("remote_version")
+	return nil
+}