apiUrl config setting shouldn't require trailing slash

If the client's `services[0].apiUrl` config setting is `http://localhost:5000/api` then the client will incorrectly try to use `http://localhost:5000/token`, get a 404 response from h, and show a "Hypothesis login lost" error. `http://localhost:5000/api/` works correctly -- the client uses `http://localhost:5000/api/token`.

Expected behaviour: `http://localhost:5000/api/token` should be used regardless of whether or not `apiUrl` ends with a trailing `/`.