[#1178] The user is activated when it performs the reset password
[#1178] Admins can invite users

The invited user starts in pending state, with the password reset key set. We
still have to send an email to the user telling him/her to change the password
and log in.

I had to change authorization code to only automatically unauthorize deleted
users, not pending. This was because the users needs to be able to perform the
password reset when pending, to be able to become active.