Paginate the Artefacts list because its slow

In development, with ~3,200 artefacts, load time went from 7600ms to 280ms.
Reinstate sorting on the artefacts index
Implement artefact search
Only use the section parameter if it's non-blank

This fixes a bug with filtering when you haven't selected a section, but
because it tries to preserve a section choice, it sets a blank section.
Don't pass around an empty section param
Preserve the filter across section changes.

From a conversation with John, this seems to fits better with the way
people would search for an artefact without knowing the section it's in.
Merge pull request #72 from alphagov/speed_up_artefacts_index

Speed up artefacts index page