Silence i18n warning
Require sign-in for Movies API

- Use ActiveModel::Serializers to return authentication token following
  successful sign-in.
- Successive API requests must include a user_email and a user_token key
  in the payload
- Any requests without the user_email and user_token will be receive a
  401 status back from the server
Refactor common headers into let blocks
Add missing params to test
Use ternary for basic branching
Correct typo in comment