This controller responds to two different routes with the same URL:
* 'admin_post_index' is the route with a name that follows the same
structure as the rest of the controllers of this class.
* 'admin_index' is a nice shortcut to the backend homepage. This allows
to create simpler links in the templates. Moreover, in the future we
could move this annotation to any other controller while maintaining
the route name and therefore, without breaking any existing link.