Adds a new method, check_3pid_auth, which gives password providers
the chance to allow authentication with third-party identifiers such
as email or msisdn.
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.
Andrew Morgan