When searching for categories it is possible for
a child category to have a slug that matches the term
exactly, but will not be found by .lookup since we
don't return these categories unless the ref matches
parent:child.
Introduces a search_sort method to each hashtag data
source so they can provide their custom sort logic of
results, in category's case putting all matching slugs
to the top regardless of parent/child relationship
then sorting by text.
cad2fe6089