Add support for S3 static storage

After merging this branch, and before pushing to dokku, you'll have to:

1. Create an S3 bucket (ideally called `openair-static`) in the S3 web console
2. Add the following config variables to dokku (`dokku config:set openair KEY=value`):
 - AWS_ACCESS_KEY
 - AWS_SECRET_KEY
 - AWS_STORAGE_BUCKET_NAME

with the proper values (the API keys you should have, and the bucket name you just created)