Have ArticlesForSet subclass ContentForSet
Have ArticlesForSet subclass ContentForSet

Since they basically do the same thing, except for the
constructing the query itself.

https://phabricator.endlessm.com/T14516

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib