In which we serve user uploaded avatars with S3

Depends on https://github.com/freedomgames/Planet-Lab/pull/20

- A file is selected for upload by the user in their web browser
- The front-end is then responsible for making a request to the back-end, which produces a temporary signature with which to sign the upload request
- The temporary signed request is returned to the front-end as JSON
- The front-end then uploads the file directly to Amazon S3 using the signed request supplied by the back-end
- The front-end then issues a PUT to the back-end setting the new avatar url on the user

@wilhow @maiamcguinness @malynda @thcrock