Track related refactoring

Add roles as nested routes to track (for the track organizer role)
Allow transition from to_accept to to_reject and backwards
Split Track#valid_dates validation to many independent ones
Show all the confirmed tracks in the conference's splashpage
Add comment in admin/Tracks#toggle_cfp_inclusion
Rewrite admin/TracksController#accept spec
Add feature spec for track requests
Change 'In' to 'Room' in Tracks#index
Rewrite Track#overlapping
Refactor code in ProposalsController
Fix typos

app/helpers/events_helper.rb

@@ -41,9 +41,9 @@ module EventsHelper
   end
 
   def track_selector_input(form)
-    if @program.tracks.any?
+    if @program.tracks.confirmed.cfp_active.any?
       form.input :track_id, as: :select,
-                            collection: @program.tracks.where(state: 'confirmed', cfp_active: true).pluck(:name, :id),
+                            collection: @program.tracks.confirmed.cfp_active.pluck(:name, :id),
                             include_blank: true
     end
   end