Implement keyword based search for Admin Toolbar Search

Problem/Motivation

Currently, searches such as "add article" don't work because the label for add article contains "Content > Add Content > Article" and the url, but not the exact string "add article". It would be nice to allow keyword based search, so that a search for "add article" would return the add article link.

Proposed resolution

Replace the default search with a custom callback that uses keyword based search. It should show suggestions that match all keywords, not just some.