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.
Replace the RABL business support scheme list.