First pass at travel advice index page

A listing of all the countries that the content api knows about.

This is missing a few features like alert statuses and the recently updated list which will be added as separate stories.

Right now, you will have to run any tests using a local copy of gds-api-adapters and with `API_DEV=1`, as the new gem release cannot be pushed to RubyGems. This pull request should not be merged until the gem has been released and the Gemfile here updated.