[web] Introduce ChatThreadComposer component for selecting users to new thread

Summary:
Introducing main component used in thread creation view, that allows selecting users to new thread and displays list of found users:

The component doesn't contain callbacks implementation yet. The diff focuses mostly on UI. The callbacks will be added in following diffs.

Test Plan: The component is not used in UI yet. I tested it after introducing chat creation view in next diffs.

Reviewers: tomek, atul, abosh, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4461