Get heroku manual build working

- Remove "Deploy to Heroku" button, since no free option
- Uses https://github.com/cyberdelia/heroku-geo-buildpack.git
- If Heroku detected, database driver switched to postgis
- Retain staticfiles folder for working ./manage.py collectstatic
- app.json has the minimal config for DEBUG=False config, loads data
- Closes #50
is `no_image` an actual directive or just a way to hide the button for now? If we're just hiding it, we might as well just remove it.