Ditch poxx and localelinter for dennis

This covers documentation, the `test_locales.sh` script and adjusts how to lint locales which I think I only documented in an email a while back.

This doesn't affect production code at all.

The big reason for pulling all this out into a separate project is that I can iterate on it there without updating the bits in kitsune and fjord and other places I'm using it.

Quick r?