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)

When this is all done and pushed to dokku, files should be collected to S3 and be served from there :)