[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.
[bug 875313] Use compile-linted-mo.sh in update script

This changes the update_locales part of the update script which we
use for deployments to use compile-linted-mo.sh instead of compile-mo.sh.

This will cause the deploy logs to contain a list of the files that
were compiled and the ones that are BUSTED. For now, we'll have to check
that for .po files that have errors that need to be fixed.
Adjust test_polint to be less stdout-spammy