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)