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
I don't think it's a good idea to not show the matching account message if there is already a user on MDN with the same email address. And it's really, really not a good idea if we're going to prevent them from creating an account with that Persona account because there's already a matching email address.