Add a property to filter facets which indicate whether they apply to annotations
and replies or only threads. Use this property to apply the filter either to all
annotations or only the root annotation in a thread respectively.

Queries based on the user or annotation body for example can apply to both.
Queries based on a selector such as page number, book chapter or quote only make
sense as thread-level filters.

Fixes https://github.com/hypothesis/support/issues/158