Stop using django.contrib.comments

First off, django.contrib.comments is being unbundled from Django and removed in Django 1.8 (and already raises deprecation warnings in Django 1.7). Second, it's overkill for what we're using it for.

Instead, replace it with a simpler model that can live in the jobs app and store the basic information it needs (sender and message, associated with a particular job posting) and use that to handle comments back and forth about job postings.