Refactor atom feeds to use common partial for entries

This involved adding a new presenter class for the index views so that the data for the index view quacks like the data for the individual country view.
Change presentation of atom summary now it's plain text.

Use simple_format to create some structure.
Remove old helper that's now unused.
Add content link to atom summaries

Govdelivery isn't pulling out the atom link and is only using the body in email alerts
Not all countries are countries. Some are territories
Merge remote branch 'origin/fix-country-terminology'
Merge pull request #282 from alphagov/ta_change_description_update

Change presentation of travel advice atom summary now the field is plain text
Remove route for video part of guide

This functionality was removed a while ago.  with this route in place, a
MissingTemplateException is caused when hitting this route.  Removing
the route will make it behave like any other non-existent part of a
guide, and redirect to the start of the guide.
Merge pull request #284 from alphagov/remove_video_part_route

Remove route for video part of guide
Attempt to simplify fetch_artefact by breaking out a few methods
Move logic for fetching artefact out into new class

There were a lot of methods cluttering the ApplicationController that were solely
about fetching and verifying a new artefact. The code becomes simpler if we pull
that out as a separate class.
Change signature of fetch_artefact to take a slug and edition

It was odd to have this method take some arguments explicitly and some implicitly
(via the params hash). It feels much cleaner to just pass them all in here. This
also makes the method a simple wrapper around the ArtefactRetriever class.
Pass logger and statsd into ArtefactRetriever
Move two exceptions that are specific to retrieving artefacts

Since these exceptions are specific to artefact retrieval they can live within
the relevant class.
Reformat for line length
Allow artefact format of custom-application so this can work for travel advice
Migrate travel advice index to use fetch_artefact method
Migrate travel advice index to use fetch_artefact method
Initial test coverage for ArtefactRetriever
Merge branch 'master' into refactor-artefact-fetching

Conflicts:
	app/controllers/application_controller.rb
	app/controllers/travel_advice_controller.rb