Requests for comment/Notifications in core

This is a followup to, and proposes that we move some of the code currently in the Echo extension into MediaWiki core.

Background
There was a successful RfC to implement a notifications framework, and the implementation happened in an extension (Echo). Since then there's been further work in the extension, and certain features of it have become integral in how editors use the wiki (mentions, userrights notifications, etc.).

Problem
Being in an extension, Echo has to use ugly hooks to suppress some core features and has generally weird interactions with some things. Furthermore, extensions that wish to use it have to do conditional checks everytime they'd like to trigger a notifications.

Finally, and most importantly,  is a reserved word in PHP, preventing us from using it as the namespace for a PHP class.

Proposal
Move the infrastructure of Echo and some notification types into MediaWiki core. Specifically:
 * flyout code
 * meta=notifications API module
 * Special:Notifications no-js page
 * formatter system for creating notification types
 * email notifications (HTML and plain text)
 * orange indicator thing for new talk notification (and get rid of OBOD, even for anons)
 * underlying code for all of the above to work
 * Some notifications:
 * user-rights
 * welcome
 * edit-user-talk
 * reverted
 * emailuser
 * cross-wiki notifications

This would leave the following in Echo:
 * page-linked
 * mention
 * thank-you-edit
 * A back-compat layer for things that haven't transitioned to core notifs yet

And while we do the migration of code, we should make sure that the code being newly introduced is architected as we'd like it. (i.e. maybe EchoEvent::create API would be different)

Note: This RfC intentionally avoids the question of. I think we do need to unify them somehow, but don't think it should block moving Echo into core.