Fixed #259 -- removed the rest of temporary Heroku config

This reverts changes from [this commit](https://github.com/django/djangoproject.com/commit/da3f2b088ab801bf84688ff355206c859a564eeb), except:

- heroku_wsgi.py -- someone deleted this already
- common_settings.py changes - they're actually quite handy for local setup if you don't have secrets.json file
Ah, sure! :)