Both iCalendar routes were unusable:
* `/conferences/.../schedule.ics` raised
`NoMethodError: undefined method 'icalendar_proposals'` because the
`ConferenceHelper#icalendar_proposals` helper isn't auto-included in
controllers; the action crashed before rendering.
* `/calendar.ics` silently redirected to `/`. The controller goes
through `load_and_authorize_resource`, which authorises `:calendar`
on `Conference` and is rejected for anonymous visitors because no
ability grants it — CanCan's `AccessDenied` rescue redirects to
root.
The helper is invoked through the controller `helpers` proxy so it
works in both contexts, and the calendar action is opted out of
resource loading and explicitly marked as not requiring authorization
since it serves a public feed across conferences. The helper is also
hardened so that proposals missing a room, difficulty level, track,
event type or scheduled time produce a valid event instead of
crashing the whole feed.
The single query was producing a tremendously large ActiveRecord allocation.
While this isn't nearly as cool as one query, it's still a significant
reduction in SQL trips, and now has the bonus of only loading what is
actually going to be displayed.
