Lowercased emails

When using this app last night, my phone autocorrected my email to have an uppercase letter in it, and the app told me it didn't recognize my email. Emails are not case sensitive, so it shouldn't matter whether or not a letter or two is capitalized. This branch lowercases all incoming email addresses as well as includes a User class method to lowercase existing emails so whether or not you input EXAMPLE@example.com or example@example.com, the app will recognize your email.

All tests pass.

WARNING: upon deployment, User.lowercase_existing_emails should be run to make sure everything works properly.