FEATURE: Add assistant to quick search widget (PR #13650)

Removes the autocomplete behaviour for categories on the search input and adds two special cases for now.

First case is for categories, typing # will display the top 10 categories of the site. The second case is for the special filters that use the in: prefix.

This is a first step, in future PRs we should add support for more special keyboards (like status:) as well as better search results while using the category shorthand.

GitHub

This is very nice! Since it’s a fair bit of code let’s be sure to merge at a safe time when things are stable.