auth: Add support for Azure Active Directory authentication.

This takes advantage of all of our work on making the
python-social-auth integration reusable for other authentication
backends.

zerver/views/auth.py

@@ -319,6 +319,7 @@ def start_social_login(request: HttpRequest, backend: str) -> HttpResponse:
     if (backend == "github") and not (settings.SOCIAL_AUTH_GITHUB_KEY and
                                       settings.SOCIAL_AUTH_GITHUB_SECRET):
         return redirect_to_config_error("github")
+    # TODO: Add a similar block of AzureAD.
 
     return oauth_redirect_to_root(request, backend_url, 'social')