feat(event): add type

Users want to specify which category their event belongs to, e.g.
- party
- sport
- family
- music/festival
- organization(al)/corporate

See:
- https://schema.org/Event#subtypes
- https://github.com/maevsi/sqitch/commit/6a479afa7993a9b64e8404ce5fd0c898ef6abd7f