Debounce search-as-you-type in post selector

[mboynes]

In the post selector, searches happen immediately (after the 3rd character), which can cause many unnecessary requests. The search REST API endpoint can be quite slow if there's a lot of content on the site, so this could end up being a significant issue. It would be great to debounce this so the search doesn't happen until the user stops typing for 2-3 seconds.

See GIF below.