'Groups'

- Collections aren't quite a group.
- teachers want groups as a proxy for classes. 
- groups could be the basis of contracts, too?
- group types? ('individual'? / school or district / contract)
- groups can belong to other groups? So a teacher's groups all belong to a school group?
    - though perhaps we only allow 1 level of nesting?
- group role: none (member but no access) / viewer / editor