Allow multiple uploads (not working fully).

@allspiritseve could you help me out with this? I'm having an issue on the Rails side, where even though multiple files get uploaded to S3, only one of the filepaths gets saved in the db. I think it's because I'm using the wrong form option - https://github.com/UM-USElab/gradecraft-development/blob/master/app/views/submissions/_form.html.haml#L22 - but I don't know what I *should* use.