[web] Fetch loggedInUserInfo in useExistingThreadInfoFinder

Summary:
This diff should be a no-op. In a later diff, I'm going to update createPendingThread so that it needs the viewer's username, as we may need to fetch their ENS name.

Depends on D6755

Test Plan: Flow, and playing around with pending threads on web

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6756