Lang, Activities, Themes

- Additional language fixes (Loader, Modules)
- Clean up Activities module, remove most of the use of $this->db from
the controller, reduce calls to has_permission() by checking the most
common permissions in the controller's constructor, storing them in
controller properties, and passing the result to the views
- Setup themes to allow admin and default themes to share files, such as
the footer (Admin_Controller, themes). Set the default theme as the
fallback when setting the theme in Admin_Controller