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/views/conferences/show.html.haml

@@ -45,14 +45,14 @@
       %section#program
         = render 'schedule_splashpage'
 
-    - if @conference.program.cfps.for_tracks.try(:open?) && @conference.splashpage.include_cfp
-      %section#callfortracks
-        = render 'call_for_tracks'
-
     - if @conference.program.cfp_open? and @conference.splashpage.include_cfp
       %section#callforpapers
         = render 'call_for_paper'
 
+    - if @conference.program.cfps.for_tracks.try(:open?) && @conference.splashpage.include_cfp
+      %section#callfortracks
+        = render 'call_for_tracks'
+
     - if @conference.venue and @conference.splashpage.include_venue
       %section#venue
         = render 'venue'