[web] Support chat creation in ChatMessageListContainer

Summary:
The diff extends ChatMessageListContainer with chat creation state.
When user enters chat chreation, the component uses useExistingThreadInfoFinder with pending personal private thread set as baseThread - so the behavior is the same as in native app.
Also, in thread creation mode, thread top bar, message list and input bar are displayed only if any users has been selected (so some thread from existingThreadInfoFider could be matched)

Test Plan: Currently there is no way in the app to enter the view by UI. The component can be displayed after entring url http://localhost/comm/chat/thread/new/.

Reviewers: tomek, atul, abosh

Reviewed By: tomek

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4465