Save yaml for competition

Resolves issue #508 

## Functional review

- [ ] Create a competition
- [ ] Go to its detail page
- [ ] There should be a "Download YAML" link on the top

## Tasks
- [x] model
- [x] unpack saves yaml
- [x] competition edit form has link to download YAML
- [x] view for downloading
- [x] test view returns proper contents
- [x] test view permissions (admins OR creator)
- [x] test view with non existent pk