Support for concurrent editing users

There is no UI lock mechanism if two users edit the same page at the same time - the data from the first user saving will be persisted.

The second user will trigger an error message, his changes will not be saved - this is not shown in the UI. This scenario should be prevented - the second user should not be allowed or at least be warned before he starts to edit a node.

No files, tests, or entry points are named. Start by reproducing concurrent editing in the UI and trace how the second save failure is surfaced. Done means the second user is prevented from editing or warned before editing, with the chosen behavior reflected in the interface.

Written by the indexing model from the issue text.