Add "new and returning attendees" report

[nbudin]

We'd like a report that allows us to determine which attendees have never been to one of this organization's conventions before, and which have.

@dkapell wrote a SQL query that does this:

select u.id, u.email, count(ucp2.*) from users u
    inner join user_con_profiles ucp1 on u.id = ucp1.user_id
    inner join user_con_profiles ucp2 on u.id = ucp2.user_id
    inner join conventions c1 on c1.id = ucp1.convention_id
    inner join conventions c2 on c2.id = ucp2.convention_id
    inner join tickets t1 on t1.user_con_profile_id = ucp1.id
    inner join tickets t2 on t2.user_con_profile_id = ucp2.id
where 
    ucp1.convention_id = 205 and
    c1.organization_id = c2.organization_id
group by u.id
having count(ucp2.*) = 1

We should take this and turn it into a report page usable from the UI, hopefully using ActiveRecord syntax instead of raw SQL.