[bug 857416] Fix infinite redirect problem

This sets DEV_LANGUAGES in local.py-dist because there are cases where
it doesn't get set in funfactory.

This also slims down locale.py-dist, updates the documentation, and
clarifies how things are set up.

I submitted https://github.com/mozilla/funfactory/pull/49 to fix funfactory, but 
we need to fix this here now.

r?