Better riverid handling

* Solves the majority of ushahidi/Crowdmap#143 and probably ushahidi/Crowdmap#159
* RiverID's are registered as soon as a new user is created
* Admins can no-longer change email addresses since that would mess with the river id association.  (w/ riverid enabled)
* Admins can no longer set passwords for users during creation (w/ riverid enabled)
* Fix RiverID error message getting 'auth.password' added to it