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/index.html.haml

@@ -24,7 +24,7 @@
           %tr
             %td
               = track.id
-            %td{style: "padding: 15px 0px 0px 10px;"}
+            %td{ style: 'padding: 15px 0px 0px 10px;' }
               = link_to admin_conference_program_track_path(@conference.short_title, track), class: 'btn' do
                 %span.label{style: "background-color: #{track.color}; color: #{ contrast_color(track.color) }"}
                   = track.name
@@ -60,7 +60,7 @@
               - else
                 = track.state.humanize
             %td
-              .btn-group{role: "group"}
+              .btn-group{ role: 'group' }
                 - if can? :edit, track
                   = link_to 'Edit', edit_admin_conference_program_track_path(@conference.short_title, track), class: 'btn btn-primary'
                   - special_style = true