(bug 44636/bug 44689) Deleted properties handling by the JavaScript UI

The UI will no longer fail if there are Snaks using properties which have been deleted.
Also allows to still edit (as far as possible) and remove Claims and References with
Snaks using delete Properties.

Change-Id: Id619259d5b29ba9b1a4d89437b21fdc362d2946b
Merge "(bug 44636/bug 44689) Deleted properties handling by the JavaScript UI"