[Channel Manager][Forward port] Save draft

Add "Save draft" button. See linked issue.

As a user
when I am editing a document
and I have made changes
and I save the draft
then the draft is saved
so another user is able to edit that draft.

As a user
when a document has been saved as draft
and I (traditionally) save the document
then the user is presented with a dialog
so it's clear that the draft changes are lost