[web] Use search users in chat composer

Summary:
We want to use search users in chat composer so can create chat with users not in user store

Depends on D8012

Test Plan: Search for users. Check if there are users not in user store. Check if they can be selected properly

Reviewers: kamil, inka

Reviewed By: kamil

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D8013