Fixing 'invalid' email address regex

I think this resolves #948. The email validation regex is expecting only lowercase characters, so complains when anything uppercase is presented.

Making the regex case-insensitive means that I can log in with an email address containing uppercase characters, but I don't know if this is really addressing the issue, or just hiding something deeper. Don't know enough about the openbadges codebase to really say, so would probably make sense for @stenington/@brianloveswords/@toolness to comment.