[web] Prevent user from closing the tab with unsaved edits

Summary: Display confirmation alert when the user leaves the page with unsaved edit.

Test Plan:

• Entered edit mode,
• Closed tab without making any changes -> no alert,
• Closed tab with some changes -> alert displayed,
• Exited edit mode, checked if the alert is not displayed after closing the modal.

Screenshot:

Reviewers: michal, inka

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7849