Better fix for autocomplete blur (select on mousedown instead of delaying blur)

Summary: Ref T10163. I would still sometimes not get a replacement after clicking with the delayed blur. This seems to fix the issue more consistently: instead of listening for a click event (which fires after the blur), listen for a mousedown event (which fires before the blur).

Test Plan: Observed consistent selection via mouse locally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15032