Email registration is unecessarily case-sensitive

[ml-evs]

i.e., we should sanitise all emails to be lowercase on sign-up.

[ml-evs]

Ugh, apparently not that straightforward:

RFC5321 mandates that the local segment of the email (i.e., before "@") should be case-sensitive, but basically every email provider treats it as case-insensitive: https://datatracker.ietf.org/doc/html/rfc5321