[web] Handle new messages when in edit mode

Summary: If the user edits the message, and new messages appear, we want the modal to stay in place. I blocked scrolling to the bottom when the user is in the edit mode.

Test Plan:

• Opened the same thread in two browsers.
• Entered edit mode in one.
• Send a message from the second browser.
• Checked if the modal stays in place.

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7850