Add the Travel Advice format

This branch adds support for travel advice content from the content api.

Whilst the travel advice format is very similar in structure to a standard content api artefact, a separate controller is used here to prevent adding further complexity to RootController. There will also be further travel advice-related actions added in the future, such as the country list.

As part of this work, I've moved the set_slimmer_artefact_headers out into the ApplicationController, joining the pre-existing fetch_artefact method. I'm not convinced ApplicationController is the best place for these methods and it might be a good idea to move these out into a module in the future.

There are also some changes to the guide navigation partial - a bool must now be provided to the render method as a local variable to specify whether part numbers should be displayed or not, as travel advice content does not refer to parts numerically.