Replace tag-listing functionality.

This results in a fair bit of duplicationg between the new URLHelper object
and the old URLHelpers module; we can remove that once we've removed the _tag
template from RABL.

Cherry pick conflicts:

	govuk_content_api.rb

We don't want local transactions presenter. We don't need it.
Display null parent field if tag has no parent.

This was the behaviour of the API before de-RABLing; if we do change it, we
should do it deliberately.
De-RABLify the artefact list.

As part of this, I had to add an extra (optional) parameter to the URLHelper
class to supply a Plek instance to look up apps. In development, we link to
the individual apps' rendering pages, so we don't have to spool up a router on
development machines, so we need to look up the URL for an artefact's
rendering app.
Replace the single tag view with a de-RABLed one.
Fix for undefined single_result_presenter
Make ResultSetPresenter reflect the way its used.

Allowing a callable or a class was an unnecessary customisation, given we were
using it in the same way every time. Similarly, allowing these to be omitted
entirely wasn't necessary.

Cherry pick conflicts. I deleted stuff, it needs to stay deleted:

	govuk_content_api.rb
Replace the tag types RABL view with presenters.
Replace the artefact RABL view with presenters.

Well, this was fairly horrendous, simply because the artefact view does so
many different things depending on the type of artefact. This is mostly a
one-for-one replacement of the RABL code, with a couple of exceptions:

  * The travel advice index is no longer bundled into the same template as the
    standard artefact, because we can do exactly the same thing with a new
    presenter that wraps around the standard one.
  * There's now a GovspeakFormatter class that handles Govspeak rendering and
    interpolating facts from the Fact Cave. This replaces the
    `process_content` global helper.
  * Some of the sub-parts of artefacts are farmed out to their own presenters;
    others aren't. There isn't any particularly reasoned approach as to which
    is which, and they're all liable to move around as needed.

Cherry pick onflicts:

	govuk_content_api.rb
	lib/artefact.rb
	views/_fields.rabl
	views/_tag.rabl
	views/artefact.rabl
Delete irrelavent files.

licences and travel advice have no purpose for odi.
Include odi field

Now that we've copied across the slug field that odi is expecting,
we are ok to delete the rabl view.
ODI does not use combined tags
Use correct related artefacts

live_tagged_artefacts was from travel advice stuff, not used by odi.
Delete stuff we don't need.
New ODI specific presenters

Author, node and organizations should now be catered for with their own presenters.