18F/hub-pages-private submodule at pages/private

Pages from pages/private will not be generated for public version of the Hub.
Make JoinerImpl.filter_private_pages idiomatic

Pointed out by Aidan Feldman in:
https://github.com/18F/hub/pull/1/files#r21643733

I'm still a fan of explicit returns, but perhaps in function this
uncomplicated, I'll make a concession to language idioms.
Merge pull request #1 from 18F/private-pages

18F/hub-pages-private submodule at pages/private