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
The provider name would be "GitHub". I'd suggest to use the provider id instead, as that's the technical name of the backend. So basically `account.get_provider().id` instead. I guess it would make sense about to use `{% set provider = account.get_provider() %}` and then use `{{ provider.name }}` and `{{ provider.id }}`.