Concurrent checklist editing

Issue Checklist app supports simultaneous editing of the checklist in multiple windows of the browser.

This way several users can work on the same checklist at the same time.

Adding new item, toggling an item, editing item's summary or reordering items does not overwrite concurrent actions performed in the other windows.

Only the same action performed on the same item overwrites concurrent change, e.g., modifying the text of the same item in different windows.

Changes made in one window become visible in other windows when any checklist action happens in those windows (e.g., checking an item gets the latest version of the checklist with changes from other windows).