Enforce lowercase email addresses in more places

Enforce lowercase email addresses in more places

We don't want two different accounts to exist with the same email
address only differing in case. This had already happened for a few
which have been manually fixed, since it turns out we only enforced the
rule on new account creation, not when accounts changed email address or
when they were created using oauth. Also add database level constraints to
make sure this cannot happen again if some codepath is missed.