Ataki/gridfs

@stvstnfrd to review commit
I'm not sure how I feel about actually creating and deleting a large
file on disk. What if an Open EdX instance increases the upload limit to
1GB? We presumbably shouldn't create a 1GB file each time tests are run.
At the least, we'll want to `patch` the `settings.` value to some
smaller value (or maybe override the value in `cms/envs/test.py`?). That
said, I wonder if we can `Mock` the upload action to avoid writing to
disk entirely.