Disable /register/available if registration is disabled (#6082)

changelog.d/6082.feature

@@ -0,0 +1 @@
+Return 403 on `/register/available` if registration has been disabled.

synapse/rest/client/v2_alpha/register.py

@@ -333,6 +333,11 @@ class UsernameAvailabilityRestServlet(RestServlet):
 
     @defer.inlineCallbacks
     def on_GET(self, request):
+        if not self.hs.config.enable_registration:
+            raise SynapseError(
+                403, "Registration has been disabled", errcode=Codes.FORBIDDEN
+            )
+
         ip = self.hs.get_ip_from_request(request)
         with self.ratelimiter.ratelimit(ip) as wait_deferred:
             yield wait_deferred