Sub bundles

Resolves issue #497

## Overview

Gives you the ability to create an organizer dataset that can contain many smaller datasets

## Tasks

- Form
    - [x] Validate data file OR data bundle, not both
    - [x] Javascript when bundle of data files given: confirm it's okay to remove data file
    - [x] Javascript confirm when single file mode but some data is in multi mode, and removes data in multi mode
    - [x] Dataset lookup + selection
    - [x] Require single data file in single mode
    - [x] Require at least 1 file in multiple data mode
    - [x] Filter queryset for sub data files
    - [x] Swap between single bundle/multibundle
    - [x] Whichever is chosen at time of submission is the choice, clear the other field before submitting. (i.e. selecting single new dataset clears many dataset selector when upload button is hit)
- [x] When saved write metadata file pointing to all datasets associated with it
- [x] Dataset download modified to tie together multiple datasets and deliver them as one
- [ ] Can reference dataset in `competition.yaml` and it unpacks fine
- [x] Everything to do with single bundles still works