Add panopticon support for `primary_section` and `sections`.

See https://github.com/alphagov/panopticon/blob/master/app/controllers/artefacts_controller.rb#L193
for final usage.

Deprecated `section` in favour of the split fields.

Cannot currently find anywhere that `section` is
still used.
Merge pull request #230 from alphagov/support-browse-pages-in-panopticon-registerer

Add panopticon support for `primary_section` and `sections`.