Set the save_file as required

It wasn’t required since the serializer sees the nullable and emptyable (??) status from the model.
No need to pass lab proxy id
Need to pass location kwargs to view
Fix UserSave tests

- use SimpleUploadedFile to create temp file
- use http bad request for post error
- use http no content for updating save file
Fix CreateUserSave view

- fetch user from request object
- use location from kwargs
Remove unused imports, add ParserMixin to views.