[web] Fix thread creation from media

Summary:
This solves ENG-5145, a regression introduced in D7434.

The issue is that pending threads don't appear in the ThreadStore. Instead of trying to pull the ThreadInfo from the ThreadStore, we should drill it through from the callsites.

Test Plan: I confirmed that I'm again able to create a thread from media in my local environment (the bug no longer repros)

Reviewers: bartek, atul, tomek

Reviewed By: bartek

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9545