Fix rendering of relatable artefact list.

This was failing due to an inefficient query against Mongo, which is
slow most of the time and breaks completely if there are enough
artefacts.

See the pull request on the `govuk_content_models` repo for details
<https://github.com/alphagov/govuk_content_models/pull/101>
Merge pull request #99 from alphagov/fix-relatable-artefacts-list

Fix rendering of relatable artefact list.