Enable pagination for the artefacts index

Now that the number of artefacts in the system has significantly
increased, this endpoint needs to be paginated to return results in a
timely fasion.
Return 500 items per page for the artefacts index

This is a good balance of page size to number of pages.  This results in
each page being around 90kB, and around 180 pages with the current data
set.
Use a batch insert to improve test performance
Merge pull request #177 from alphagov/artefacts_index_pagination

Paginate the artefacts index.