Move gallery_ready(), maintenance_mode(), and private_gallery() calls to Controller::before().

- add Gallery_Controller which extends Kohana_Controller (so it is inherited by every controller)
- add Controller::before() which calls the three functions above.
- change ALLOW_PRIVATE_GALLERY and ALLOW_MAINTENANCE_MODE to Controller class variables.
- use the newly-changed variables in maintenance_mode() and private_gallery()
instead of ReflectionClass calls.
- remove Gallery::ready() from index.php.
- add "is_admin" parameter to admin-specific routes in bootstrap.
- use "is_admin" to set Theme::$is_admin, and call it later when needed.
- change Theme::load_themes() to use current request, not initial.
- add todo notes for K3 login rerouting.