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.

spec/controllers/conferences_controller_spec.rb

@@ -49,4 +49,25 @@ describe ConferencesController do
     end
   end
 
+  describe 'GET #calendar' do
+    it 'returns iCalendar data for all conferences' do
+      get :calendar, params: { format: :ics }
+
+      expect(response).to be_successful
+      expect(response.content_type).to start_with('text/calendar')
+      expect(response.body).to start_with('BEGIN:VCALENDAR')
+    end
+
+    it 'returns iCalendar data with full schedule when full=true' do
+      conference.program.update!(schedule_public: true)
+      create(:event_scheduled, program: conference.program)
+
+      get :calendar, params: { full: true, format: :ics }
+
+      expect(response).to be_successful
+      expect(response.content_type).to start_with('text/calendar')
+      expect(response.body).to start_with('BEGIN:VCALENDAR')
+    end
+  end
+
 end