[web] Introduce pending thread creation in websiteResponder if it is missing

Summary:
The diff introduces mechanism for setting pendingThread in navInfo while first page load,, so it match activeChatThreadID value.
The reason is to load pending thread correctly in navigation after page refresh - when we know only pending thread ID from URL.
The solution doesn't work for sidebars right now. Creating pending sidebar would require addidtional data (like parent thread info) that should be found efficiently using only sourceMessageID, and it could be introduced in the future.

Test Plan: Run web app and using search find user, that has no chat with our user. Clicking that user will create pending personal chat. After refreshing the page, the pending chat should be loaded automatically and user is not redirected to first chat from list.

Reviewers: tomek, atul, abosh

Reviewed By: tomek, atul

Subscribers: ashoat, adrian, abosh

Differential Revision: https://phab.comm.dev/D4455