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
Can we add that this mocks what Etherpad would do when an author that has made an edit leaves?