auth: Add logic for avatars to the GitHub auth email selection page.

For the emails that are associated to an existing account in an
organisation, the avatars will be displayed in the email selection
page.  This includes avatar data in what is passed to the page.

Added `avatar_urls` to the context in `test_templates.py`.

zerver/tests/test_templates.py

@@ -181,6 +181,7 @@ class TemplateTestCase(ZulipTestCase):
             request=RequestFactory().get("/"),
             invite_as={"MEMBER": 1},
             max_file_upload_size = 25,
+            avatar_urls={"john@gmail.com": "www.zulip.com"},
         )
 
         context.update(kwargs)

zproject/backends.py

@@ -38,7 +38,7 @@ from social_core.exceptions import AuthFailed, SocialAuthBaseException
 
 from zerver.lib.actions import do_create_user, do_reactivate_user, do_deactivate_user, \
     do_update_user_custom_profile_data, validate_email_for_realm
-from zerver.lib.avatar import is_avatar_new
+from zerver.lib.avatar import is_avatar_new, avatar_url
 from zerver.lib.avatar_hash import user_avatar_content_hash
 from zerver.lib.dev_ldap_directory import init_fakeldap
 from zerver.lib.request import JsonableError
@@ -658,10 +658,17 @@ def social_associate_user_helper(backend: BaseAuth, return_data: Dict[str, Any],
             chosen_email = backend.strategy.request_data().get('email')
 
         if not chosen_email:
+            avatars = {}  # Dict[str, str]
+            for email in verified_emails:
+                existing_account = common_get_active_user(email, realm, {})
+                if existing_account is not None:
+                    avatars[email] = avatar_url(existing_account)
+
             return render(backend.strategy.request, 'zerver/social_auth_select_email.html', context = {
                 'primary_email': verified_emails[0],
                 'verified_non_primary_emails': verified_emails[1:],
-                'backend': 'github'
+                'backend': 'github',
+                'avatar_urls': avatars,
             })
 
         try: