Improve Subject Filter in Directory

[SteelWagstaff]

At present, we store and display the Thema subject term for each book in the Subject facet. Even though Thema is a highly structured taxonomy, no discernable relationship between subject terms is visible in our Directory. We also display a whopping 832 subject terms, which is so overwhelming as to be almost useless.

The goal is to replace the current subject terms with a flattened Thema taxonomy structure which includes at least the first and second levels. This will keep the list short and manageable when the facet is opened and the subjects have not been expanded. The facet search function should still function for third and fourth levels, even though they are invisible (e.g., a search for third-level subject Television will produce its second-level parent, Performing Arts).

See: https://docs.google.com/spreadsheets/d/1kqrhUi2sxTWNTJJ0_o7LeFjhhHu9L1D8/edit#gid=271788788

Top level taxonomy items would display by default with the total count of items in all child subject categories, etc. Each top level taxonomy should have a + button which allows it to be expanded and which would then display any second level categories which have subject terms. When expanded, a top-level subject might look something like this:

We could potentially repeat this at the third and fourth level (which have 1087 and 1862 possible headings, respectively), but let's start with just top 2 levels for now (20 terms in top level, and 145 in second level).