Reorganise travel-advice tests to group index ones together
Update index tests to use json fixture with new format

Currently failing as expected
Update index atom tests to use json fixture
Update controller and views to use the new artefact format.
Setup related items on travel-advice index
Create presenter for travel-advice index
Update index view to use data from artefact.
Move loading and sorting of countries into presenter
Merge pull request #235 from alphagov/travel_advice_top_level_artefact

Travel advice top level artefact