[web] Add fake pending thread during chat creation

Summary:
Context: here

Adding pending thread mock with "New thread" title, when we haven't chosen any users yet in chat creation mode.

Test Plan:
I. Appearing:

During creating new chat:

1. Go to web
2. Click on Create new chat
3. Mock with "New thread" title appear

Via url:

1. Go to /chat/thread/new/ subpage (or refresh page, when you are already on it)
2. Mock with "New thread" title appear

Via removing all selected users from thread composer:

1. Go to chat creation mode
2. Add any users
3. Remove them
4. Mock with "New thread" title appear

II. Disappearing:

Via selecting any users:

1. Go to chat creation mode
2. Select any users
3. Active mock thread changes to pending or already existed thread

Via closing chat composer:

1. Go to chat creation mode
2. Close thread composer
3. Mock with "New thread" tittle disappear

III. Additional:

• Mock thread works exactly the same as normal pending thread in ChatThreadComposer, especially during selecting and removing users or switching in creation mode between pending and already existing threads.
• Improved thread list behaviour (after closing ChatThreadComposer) is implemented in next diff (D4940)

Reviewers: tomek, jacek, atul

Reviewed By: tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D4939