Add an app to update the email signature PNG daily.

Created by: YPCrumble

@lwm in the spirit of moving quickly, this is a preliminary PR for adding the email signature png.

The path should be:

https://api.carbondoomsday.com/media/carbondoomsday_email_signature.png

I wrote a couple tests for the Python portion of the implementation, but I'm less confident about my celery and dokku integrations...and not sure how to build out the staging server so they're untested. Would love any feedback and happy to make adjustments as required. Thanks!