Add organisations or schools as a separate group

[skade]

@schoolofma suggested in #42 to add schools and groups as a separate kind of entities.

I think that makes sense, even if we don't display it yet: if we're doing grouping, we shouldn't shoe-horn too many in the pre-existing groups.

[PragTob]

I think organisations is fine, I'd wait with a separate schools group until we got more schools