[Fix bug 1076865] Add alternate email addresses in ExternalAccount.

* Add TYPE_EMAIL in ExternalAccount model.
* Exclude TYPE_EMAIL from external account formset.
* Exclude TYPE_EMAIL from accounts property.
You also need to exclude the `TYPE_EMAIL` from the list of choices here [0].

[0] https://github.com/mozilla/mozillians/blob/master/mozillians/users/models.py#L822