[web] Introduce "Create new chat" button

Summary:
Primary button allowing to create new chats on the bottom of chat list.
Disabled if already in chat creation mode.

Test Plan: Click the button, new chat creation process (with empty user list) should be started.

Reviewers: tomek, atul, abosh

Reviewed By: tomek, abosh

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4469