[web] Make ChatThreadComposer search bar auto-focus

Summary: While working on D7388, I noticed that I had to do an extra click to get to the search bar after opening up the composer. I figured we can improve this experience by auto-focusing it.

Test Plan: Make sure ChatThreadComposer auto-focuses when open, both by pressing the "Create new chat" button, but also when loading it directly by navigating to http://localhost:3000/comm/chat/thread/new/

Reviewers: inka, tomek

Reviewed By: inka

Subscribers: atul

Differential Revision: https://phab.comm.dev/D7389