Case-insensitive email on the signup form

[markspolakovs]

The sign-up form validates emails using the regular expression ^[a-z]{1,6}[0-9]{1,6}$, which rejects upper-case letters. This means that when someone signs up and enters their email with upper-case (like some iOS devices have a habit of doing), they get a strange "your email is invalid" error.

We should relax this - either by permitting upper-case characters there, or by lower-casing the email before validating it (using strings.ToLower) - my preference is for the latter.