Require CAPTCHA for guest URL submissions

- Add recaptcha gem.
- Add application helper methods for resource_name, resource, and
  devise_mapping in order to use sign in and sign up links in Screenshot
  tab.
- Display modal with CAPTCHA form to guest users after attempting to
  submit a URL.
- Don't allow guest users to view screenshots.
- Update application.yml example configuration to fix a typo and include
  OmniAuth API keys, Amazon Web Services API keys, and reCAPTCHA API
  keys.
- Separate guest and registered user Cucumber features for URL form.
- Obscure screenshot filename.
- Add extra JavaScript for cleaning URL and submitting upon pressing
  enter key.