This implements this very crudely: this probably isn't viable
because parting a user from all their rooms could take a long time,
and if the HS gets restarted in that time the process will be
aborted.
Make sure that we delete devices whenever a user is logged out due to any of
the following situations:
* /logout
* /logout_all
* change password
* deactivate account (by the user or by an admin)
* invalidate access token from a dynamic module
Fixes#2672.
Non-functional refactoring to move deactivate_account. This means that we'll be
able to properly deactivate devices and access tokens without introducing a
dependency loop.
