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
Setting `ACCOUNT_UNIQUE_EMAIL=False` isn't enough. I got:
```
IntegrityError: (1062, "Duplicate entry 'lcrouch@mozilla.com' for key 'email'")
```
Hmm ...