[web] Convert activeChatThreadItem to a hook

Summary:
This will make the next diff a little easier.

Depends on D13821

Test Plan:
Flow, and tested in combination with the rest of the stack:

1. Make sure membership operations (user joining / leaving) don't appear in MessagePreview
2. Make sure reactions to the viewer's messages still appear in MessagePreview
3. Make sure reactions to other user's messages don't appear in MessagePreview
4. Test fresh login to thread with only one message in the last 14 days, which is a reaction to a non-viewer message. Make sure the reaction initially appears in MessagePreview, but then is replaced after more messages are fetched by the client

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13822