Only recent authors can publish a collab doc

Only users who recently editted a collaborative document can publish it.
This should avoid situations where a user who is only viewing a document
triggers a publication under his name even if he did not make any
changes himself.

This relies on https://github.com/oaeproject/ep_oae/pull/3
I think it would be good if we could do `ContentAPI.emit(ContentConstants.events.UPDATED_CONTENT_BODY, ctx, newContentObj, contentObj, revision);` regardless of whether or not there is a difference with the previous revision. As this function will only be executed when a user has made an actual change, this will just ensure that our activity summaries are accurate.

This will need some tests as well.