Downcase email addresses at account creation time

[lizadaly]

We're doing a case-insensitive check at invitation time now, but @cath9 and I agree we should just go ahead and force-lower all new incoming email addresses at account creation. If someone tries to log in with mixed case that'll fail unless we also lower-cased all existing legacy addresses first. Given the number of accounts I think it's probably fine to just handle those as needed.

Originally posted by @lizadaly in #1363 (comment)

[lizadaly]

It looks like login already downcases the hostname portion, so we're talking about doing something similar for the username portion: https://github.com/harvard-lil/h2o/blob/develop/web/main/authenticator.py