FEATURE: Add assistant to quick search widget

[pmusaraj]

screencast.2021-07-13.11-21-13.mp4

This replaces the autocomplete dropdown for categories and usernames on the search input and insteads adds suggestions for the filters as items in the search results.

As seen in the screencast, typing a partial category name, i.e.shows #do will return autocomplete suggestions in the main results space that the user can select and search for.

The PR adds the same behaviour for @mentions as well as in: status: order: keywords. The following suggestions are included:

in:title
in:personal
in:seen
in:likes
in:bookmarks
in:created

status:open
status:closed
status:public
status:noreplies

order:latest
order:views
order:likes
order:latest_topic

An API function has also been included so that plugins can extend the in: shortcuts. For example, addInSearchShortcut("in:assigned") should allow the assign plugin to show its search filter when it is enabled.

[eviltrout]

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.