Split migrations folder (#21549)

There are too many files in `models/migrations` folder so that I split
them into sub folders.

models/migrations/base/main_test.go

@@ -0,0 +1,13 @@
+// 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 base
+
+import (
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	MainTest(m)
+}