Allow authentication through email token

[audiodude]

We should be able to authenticate a user using only an email address, provided that the user is able to click a link that we send to that address (which would contain an encrypted token which itself contains the email address).

Unfortunately this means we would have to implement email capabilities, presumably through Mailgun.

This also poses the question of what to do if a user logs in with Google (and has [email protected] stored in their profile) but tries to authenticate again using that email directly. Presumably it is okay to allow this authentication to proceed?