[web] Introduce useOnClickNewThread hook

Summary:
Introduce hook for entering chat creation mode with empty user list.
It will be used after pressing "Create new thread" button.

Test Plan: The callback is not used yet. Can be tested after next diff introducing button

Reviewers: tomek, atul, abosh

Reviewed By: tomek, abosh

Subscribers: ashoat, tomek, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4466