Replace the single local authority RABL template.

We can't remove the `_local_authority` partial just yet, because it's used by
the artefact fields partial when the artefact has a LocalTransactionEdition.
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.