Use storage API to access files

[markpasc]

As noted by @johanneswilm, we should use DefaultStorage (see the file storage API docs) instead of the local system's filesystem API to load and save media files, to support an alternate storage backend configured in the Django project if any.

[johanneswilm]

those two patches seem to cover it at least for me here. I am not really sure if copying files is only possible by reading and writing to them, but that's the only way I found at least.

The only trouble is the issue about stdout and file accessing.