DEV: move post flags into database (#27125)

This is preparation for a feature that will allow admins to define their custom flags. Current behaviour should stay untouched.

lib/guardian/flag_guardian.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module FlagGuardian
+  def can_edit_flag?(flag)
+    @user.admin? && !flag.system? && !flag.used?
+  end
+end