infer attendance from roll call votes at an event

[fgregg]

if an event is missing attendance, but there are roll call votes, we could infer attendance from the roll call votes.

relates to #374

[derekeder]

we tried searching for events that match this, but only one event came up from 2015:

select
  *
from
  event
  inner join eventagendaitem on event.id = event_id
  inner join eventrelatedentity on eventagendaitem.id = agenda_item_id
where
  entity_type = 'vote_event'
  and event.id not in (
    select
      event_id
    from
      eventparticipant
    where
      entity_type = 'person'
  )
limit
  10;