Hide sidebar filters with zero or one choices

Previously, the logic was inconsistent about how explore filters were hidden, leading to awkward UI elements. This adds some default text in those cases for primary headline filters, as well as hiding them in the sidebar (even if they were a primary). It also fixes a bug where a filter that's not available on load is never available.

Issue ots/clients/lfc/torque-tracker#255: Sometimes, filter set titles appear with no options

This will auto merge in 48 hours