social auth: Validate email in backends without get_verified_emails.

If the social backend doesn't have get_verified_emails emails, and we
simply grab kwargs["details"].get("email") for the email, we should
still validate it is correct.
Needed for SAML. This will get covered by tests in upcoming commits that
add SAML support.

zproject/backends.py

@@ -725,8 +725,12 @@ def social_associate_user_helper(backend: BaseAuth, return_data: Dict[str, Any],
             return None
 
         validated_email = chosen_email
-    else:  # nocoverage
-        # This code path isn't used by GitHubAuthBackend
+    else:
+        try:
+            validate_email(kwargs["details"].get("email"))
+        except ValidationError:
+            return_data['invalid_email'] = True
+            return None
         validated_email = kwargs["details"].get("email")
 
     if not validated_email:  # nocoverage