[web] Displaying modal of editing message GUI

Summary: Added displaying editing message modal in place of the original message.

Test Plan:

• Entered edit mode.
• Checked if the background is dimmed, and the modal displayed on top of it.
• Resized window, checked if the position was updated.
• Exited edit mode.
• Checked if the modal disappeared.

Screenshot:

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7836