Refactor CoreServiceProvider

A good start I think, but still some work to do. If we go ahead with
https://github.com/flarum/core/issues/132#issuecomment-117507974 (which
I am in favour of), we can extract the entity-related stuff into some
smaller service providers (e.g. discussion repo, an event listener,
permissions, and gambits stuff could all go in
Flarum\Core\Discussions\DiscussionsServiceProvider).

src/Api/ApiServiceProvider.php

@@ -22,6 +22,8 @@ class ApiServiceProvider extends ServiceProvider
                 return new UrlGenerator($this->app->make('flarum.api.routes'));
             }
         );
+
+        $this->app->register('Flarum\Locale\LocaleServiceProvider');
     }
 
     /**