[mw.notification] Introduce a full fledged notification system into core replacing...

[mw.notification] Introduce a full fledged notification system into core replacing jsMessage.

- Support for multiple messages instead of just one is included.
- Persistent messages are also supported.
- Tagged message replacement is supported so that only one message of a type will show up at once.
- A light flood protection is included to stop all the notifications from disappearing at once.
- Watchlist uses it's own tag now.
- Legacy jsMessage messages still replace each other by using a tag: 'legacy' option
- #mw-js-message was an old id used for something with a completely different style. So rather than re-using it that id is being dropped. Skins can now style the new notifications.
- Core is no longer using messageBox and no extension uses it so instead of making a compatibility layer for it that module is just being dropped.

This is a follow up to I41c70d78.

Change-Id: I2b3d9cbce6d1514d754b4403ec878d89ea6af29d