Add pretend authenticated API for govuk-alerts

We can define the API properly in future work. I've used a separate
blueprint from "broadcasts" since this API is purely internal, and
it's helpful to make it clear it's specific to govuk-alerts.

app/authentication/auth.py

@@ -60,6 +60,10 @@ def requires_no_auth():
     pass
 
 
+def requires_govuk_alerts_auth():
+    requires_internal_auth(current_app.config.get('GOVUK_ALERTS_CLIENT_ID'))
+
+
 def requires_admin_auth():
     requires_internal_auth(current_app.config.get('ADMIN_CLIENT_ID'))