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/admin/tracks/toggle_cfp_inclusion.js.erb

@@ -5,4 +5,9 @@ track_cfp_td = $('#cfp_switch_' + track_id);
 track_cfp_value = <%= @track.cfp_active %>;
 
 track_cfp_td.attr('data-order', track_cfp_value);
+
+/*
+* The updated data-order attribute isn't taken into account
+* until we invalidate the cell
+*/
 $('#tracks').DataTable().cell(track_cfp_td).invalidate();