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
Yeah, we're into the weeds and edge cases. The first case is an issue with the way allauth detects matching accounts.

Persona is not more trustworthy than GitHub. We trust that both providers have done their own due diligence in verifying the user owns the email address. But in this case we **know** that allauth has **not** verified the user owns the email address.

1. User A adds email E to their profile
2. User A **does not verify** email E
3. User B signs in with Persona and email E
4. User B gets the "sign up" page
5. User B enters a username
6. User B is bounced back to sign up without any error message

We can add `verified=True` to this check for a matching account, and show the error message, "An MDN profile is already using that email address." But, allauth still won't let the user sign up with an email address on someone else's profile - whether or not it's been verified.

User A may not actually own that email address. User A can simply add any email address to their profile and prevent other users from signing up with it - via Persona, GitHub, or allauth.