fix bug 1057012 - show connect prompt to MDN users signing in with ...

... a Persona email address matching a GitHub account
Spot-check:
* Create an account via GitHub
* Sign out
* Sign in with a Persona email address matching the GitHub account
* [x] Verify you get a prompt about the existing MDN profile
* [x] Click the 'sign in and visit social connections' link
* [x] Connect via Persona to add the Persona connection
Just for the sake of argument, what if we'd disable enforcing the uniqueness of email addresses? That would allow adding the same email address to multiple user accounts. We then would be able to use `verified=True` to ascertain whether there was a user account that was actively used before, just adding it to an account doesn't suffice to say that. And users could continue with the signup if they decide they want to have a separate account with that email address.. or click on the connect link.