Support for 'Discard unpublished changes' in Document Workflow

We need to support discarding unpublished changes in the unpublished variant. currently you can only discard changes in a draft.

We need to also support this for branches! Therefor we need to take the following into account:

1) When discarding unpublished changes, we always discard with respect to the same branch its published version if that version is present (thus discarding 'Christmas unpublished changes' is wrt the 'Christmas published' if that published version is present
2) Otherwise, the discarding is done wrt the 'master-published' if present
3) If there master-published is not present, discard will be done again any other published version.

If there is no published version, discard unpublished changes is not available.