Add EventMinuteItem to Event, similar to EventAgendaItem

[fgregg]

AgendaItems should be for meetings that haven't happened yet, and MinuteItems should be for events that have happened.

Where possible, minutes should link to bill actions (and through bill actions to votes).

[fgregg]

we already have this a little bit through event.related_entities.vote and vote_event.bill_action_id