[web] Added message editing

Summary: Added message editing after the user clicks Save or presses Enter.

Test Plan:

• selected message to edit, made changes, pressed Save -> the message was successfully edited,
• selected message to edit, pressed save without edits -> exited edit mode without sending edit message

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7828