Don't let files be uploaded twice

We now save a hash for every file the find duplicates right when they
are uploaded.

We'll need to tell the user about this. For now it just shows the file
as being uploaded correctly, even though the response's status code
is 409 CONFLICT.
Don't import settings directly in tasks

Instead of importing marion.settings directly, we should use app.config
to have to possibility to overwrite the settings externally.