Make iCalendar feed routes return calendar data

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.

app/controllers/schedules_controller.rb

@@ -23,7 +23,7 @@ class SchedulesController < ApplicationController
       end
       format.ics do
         cal = Icalendar::Calendar.new
-        cal = icalendar_proposals(cal, event_schedules.map(&:event), @conference)
+        cal = helpers.icalendar_proposals(cal, event_schedules.map(&:event), @conference)
         cal.publish
         render inline: cal.to_ical
       end