Replace "artefacts with tag" view with presenters.

Cherry pick conflicts:

	govuk_content_api.rb
	views/with_tag.rabl
Remove the last vestiges of RABL.

And there was much rejoicing.

Cherry pick conflicts:

	Gemfile.lock
	config/rabl.rb
	govuk_content_api.rb
Fix misnamed test file.
Remove the old URL helpers.

A few needed rescuing into the new URLHelper object, as they're used by the
app class itself.

Cherry pick conflicts:

	govuk_content_api.rb
	lib/url_helpers.rb
	test/requests/local_authority_request_test.rb
Remove timestamp helpers.

Their functionality is now in the ArtefactPresenter.

Cherry pick conflicts:

	govuk_content_api.rb
	lib/timestamp_helpers.rb
Remove the defunct content format helpers.

Their functionality is now in the `GovspeakFormatter` class.

Cherry pick conflicts:

	govuk_content_api.rb
Comment the complex or inter-related presenters.

As suggested by @alext. +1 by @mnowster

Cherry pick conflicts:

	lib/presenters/artefact_licence_presenter.rb
Remove rabl from /section.json

Now powered by Section and Module presenters.