Use new API endpoint to get single contact list

When we first built the contact list feature this endpoint didn’t exist.
Now it does we can remove the slightly cludgy looping-through-all-the-lists
code.

app/models/contact_list.py

@@ -32,11 +32,10 @@ class ContactList(JSONModel):
 
     @classmethod
     def from_id(cls, contact_list_id, *, service_id):
-        # This is temporary until we have a get single list endpoint
-        for contact_list in ContactLists(service_id):
-            if contact_list.id == contact_list_id:
-                return contact_list
-        abort(404)
+        return cls(contact_list_api_client.get_contact_list(
+            service_id=service_id,
+            contact_list_id=contact_list_id,
+        ))
 
     @staticmethod
     def get_bucket_name():