Requests for comment/Cross-wiki notifications

Something about why we need global notifications

New notification possibilities
Well, at least theoretically:
 * An image you uploaded to commons was added to article XX on xxwiki
 * An article you started on xxwiki was linked to an article on xxwiki

User interface

 * In the flyout, the user can choose to see all notifications (global + local) or just local notifications. Any unread global notifications would always be shown.
 * Some kind of toggle on Special:Notifications to switch between global + local
 * Something about figuring out enotifs based on local preferences...
 * Need some indicator in the flyout that a notification is not from this wiki
 * User options on which notifications go global, so you can choose any 'mention' would be seen immediately, but a thank (no response needed) isn't (something like this?)

Backend

 * All notifications are stored in one central database table, with the current schema plus one column indicating which dbname it came from
 * API (meta=notifications) extended with parameters to fetch global notifications or just local ones
 * Integrate with CentralAuth - ideally it would work with a shared user table too:
 * Only users with a global account can have global notifications
 * If a user doesn't have a global account, they get echo notifications in the standard manner, split across all wikis