[web] Storing edited message component position

Summary: To display a modal of the editing message GUI, I have to store and update the current position of the edited message.

Test Plan:

• Checked if the position is correctly stored after entering edit mode.
• Resized the window, checked if the position is updated.
• Added/removed few lines to the message, checked if the position is updated.

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7833