Changed the '/service/unique' endpoint to optionally accept the
service_id parameter. It now doesn't matter if a user tries to change
the capitalization or add punctuation to their own service name. But
there should still be an error if a user tries to change the punctuation
or capitalization of another service.
service_id needs to be allowed to be None until notifications-admin is
updated to always pass in the service_id.
e1cc8175d7