[PP-1] Poll for other users changes to layout/model/preview using pending changes

Overview

Until we have something like Mercure for real-time updates, we need to poll for other users' changes.
We already do that via the pending endpoint.
We can include other users changes for the currently edited item in that payload - Code for this exists in the 3492065-future-steps branch.

Proposed resolution

Explore that approach

User interface changes