DB::Users: Perform case-insensitive email matching
DB::Users: Perform case-insensitive email matching

This fixes two things:
- It's not possible to create two accounts with the same mail address
  with different case (although the user+xyz@domain trick still works).
- The password reset form is now case-insensitive as well. Some people
  had problems with the case-sensitive behavior in the past.

erengy/vndb,erengy/vndb,christian-roggia/vndb,morkt/vndb,erengy/vndb,christian-roggia/vndb,christian-roggia/vndb,morkt/vndb,morkt/vndb,morkt/vndb,morkt/vndb