We update the pills typeahead logic to also include user group
results and pass the "user_group" key in `opts` to enable this
option for Add subscriber form.
The changes includes:
* Exporting the `query_matches_name_description` function, to
deduplicate the typeahead `matcher` logic.
* Creating a `people.is_known_user` function, to deduplicate
the typeahead `sorter` logic.
* Add a new `user_group_pill.js`, to allow adding user group
pills in input widgets that support pills.
This has been tested manually as well as by adding some new
node tests.
m-e-l-u-h-a-n