Setting personal email address should always be possible independent from the used backend

Please see related commit message.

As nobody responded to my proposal, I'll try it this way...
> email is a vaiue saved in the user preferences table, we do not have it bound to user backends at the moment. If you think of LDAP, we read if from the directory and save it in this place as well.

BTW: same for shibboleth - on login the email will be set by the user backend - giving the user the ability to change the email will cause confusion