Ensure user mail address uniqueness
Currently multiple users can have the same mail address. While this is not a problem in uffd, many services (e.g. Gitlab and RocketChat) don't allow different users to have the same mail addresses.
One question is how we compare mail addresses (case-insensitve?, space-normalized?, ...). It would be good to check how other applications handle this.
How should we handle duplicate mail addresses on signup?
How should we handle duplicate mail addresses when a user changes his mail address?