Extend user client to count users with permission

One of the things we need to know for a service to go live is whether
they have at least two users with the ‘manage service’ permission.

So this commit adds a method to the client to count how many users have
a given permission. We can do logic on this count later. But having the
counting done in the client feels like a cleaner separation of concerns.

Meant some refactoring of the way `service_id` is extracted from the
request, in order to make it easier to mock.

app/notify_client/user_api_client.py

@@ -131,6 +131,12 @@ class UserApiClient(NotifyAdminAPIClient):
         resp = self.get(endpoint)
         return [User(data) for data in resp['data']]
 
+    def get_count_of_users_with_permission(self, service_id, permission):
+        return len([
+            user for user in self.get_users_for_service(service_id)
+            if user.has_permissions(permission, any_=True)
+        ])
+
     def add_user_to_service(self, service_id, user_id, permissions):
         endpoint = '/service/{}/users/{}'.format(service_id, user_id)
         data = [{'permission': x} for x in permissions]