Add New Relevance feature flag

Issue

We've added a subject_rank_is solr index in galileo_admin.

We want to use that in galileo_search to make results more relevant when subject facets are chosen.

This is only partially implemented, so we need a feature flag to help us explore things in staging.

Discussion

Currently our configuration says

• Relevance: 1) score/weights descending, 2) name ascending
• Name: 1) name ascending

Note that when there isn't a query, there isn't a score, so the Relevance sort is the same as the Name sort. This is also true when you've chosen a facet with no query--i.e., there isn't a score.

I think the following is probably our target:

1. When no query or facet
   • sort by score, name (which is really just sorting by name because there isn't a score)
2. When there's a query
   • sort by score, name
3. When there's a non-subject facet (with or without a query)
   • sort by score, name
4. When there's a subject facet (with or without a query)
   • sort by score, subject_rank, name

This shows that we really only want to introduce the subject_rank when a subject facet is selected.