[bug 875313] add dennis and polib to vendor-local

This adds dennis which is a utility for linting .po files. It requires
polib.
[bug 875313] Add polint management command

This adds a polint management command which uses dennis in the django
project environment to lint .po files.
[bug 875313] Add compile-linted-mo.sh script

This uses polint to selectively compile .po files to .mo files iff
they don't have egregious errors.