For the coming features we will need to store files on the meet side.
(for instance user backgrounds).
This commits adds a new Model to manage files, and the associated
serializers & viewsets. All are tested.
This work was heavily inspired by the work done by our friends at
https://github.com/suitenumerique/drive
It build on the same architecture design (upload directly to S3 but
download goes through our proxy), but model is much much simplier
(no folders, no file sharing, etc.).
