Poor support for thousands of authors - suggest changing author dropdown to autocomplete

[rubberduck]

We have WP site with over 3,000 authors and more to be added soon. It's just a really big, popular site, with lots of user generated articles. Among many problems around author management, one issue is the fact that the author dropdown (e.g. whene editing blog posts) contains 3,000 authors, and it's a pain to always have to find which one we need. I suggest changing the dropdown to an auto-complete so that you could type into it and see results in real time, instead of needing to select from a list of 3,000 options.

[ocean90]

Related: #19810, #19867

[SergeyBiryukov]

Duplicate of #19867.