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
@groovecoder I'm afraid you have to explain the two cases in more detail as the short answer whether we can disable the email address feature in allauth is no. I don't follow how this would even solve this issue and frankly bring us back to square one in assuming Persona is somehow a safe source of identity data. Persona is in my opinion not more trustworthy than GitHub.