[web] Wait before fetching thick thread messages

Summary:
This is a workaround to the scrolling issue.

https://linear.app/comm/issue/ENG-9097/fetching-new-messages-in-message-list-is-glitchy-when-it-happens

Test Plan: Create a thick thread with 200 messages and verify that there are no scrolling artifacts.

Reviewers: kamil, ashoat, inka

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D13560