Disable the save button when there is no past history

If the user has undone all their actions they should not be able to
save the document. By updating the 'edited' property we also solve
the issue of the onbeforeunload alert displaying if all edits
have been undone.

Bug: 42939
Change-Id: Ib3bc5b0e2f21781166a17a3fdf4f5286e6713859
Merge "Disable the save button when there is no past history"