Clarify post IDs argument to `wordpressSubs`

[emhoracek]

Recently we changed the storage of post IDs for wpNoPostDuplicates from a Maybe IntSet that is stored in the WordPress b data type to Maybe (MVar (Maybe IntSet)). This is much more complex and probably a little confusing. Maybe we could try making a data type just for storing post IDs that making more explicit what the Maybes mean. So, instead of Maybe we could make something like data PostDuplicatesTracker = NotTracking | Tracking IntSet. Then we would have Maybe PostDuplicatesTracker... or something.