Merge branch 'huacnlee/current-settings-autoload-bug-in-development' into 'master'

Fix CurrentSettings autoload bug in development

_Originally opened at !2526 by @huacnlee._

- - -

In development env, when we changed any file hit Rails reload, it will failed with this message:

```
A copy of Gitlab::CurrentSettings has been removed from the module tree but is still active
```

See merge request !2753

lib/gitlab/current_settings.rb

@@ -7,8 +7,8 @@ module Gitlab
         settings = nil
         if connect_to_db?
-          settings = ApplicationSetting.current
-          settings ||= ApplicationSetting.create_from_defaults unless ActiveRecord::Migrator.needs_migration?
+          settings = ::ApplicationSetting.current
+          settings ||= ::ApplicationSetting.create_from_defaults unless ActiveRecord::Migrator.needs_migration?
         end
         settings || fake_application_settings