[web] Introduce chat-creation mode in navInfo

Summary:
The diff extends navInfo object with new chat-mode - which is an enum - 'view' or 'create', and`selectedUserList` containing list of users read from URL.
The aim is to keep URL while creating (or finding existing) thread look like /chat/thread/new/<list_of_added_users_separated_by_comma>.
So for example /chat/thread/new/1,7,48/ is going to redirect user to chat creation screen with selected three users with IDs 1,7 and 48.
Empty list will create URL look like /chat/thread/new/

Test Plan: The change doesn't affect current navigation. New paths are not used in application yet. I tested the navigation after introducing more thread creation logic (in next diffs)

Reviewers: tomek, atul, ashoat, abosh

Reviewed By: tomek, atul, abosh

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4457