Make header use *ActionPermissions

Renamed PostActionPermissions to more general RevisionActionPermissions & made
it accept any AbstractRevision object (that's where the moderation lives anyway)

Changed header code to use this class.

Made the class accept null for revision (like for checking permissions to create
a post/header)

Change-Id: Ic173c47f90b6ac1c7b7dd80befef34bdced23d66