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/_conference_details.html.haml

@@ -30,10 +30,10 @@
                 = link_to "Register", new_conference_conference_registration_path(conference.short_title), class: "btn btn-default", disabled: cannot?(:new, Registration.new(conference_id: conference.id))
                 - if cannot?(:new, Registration.new(conference_id: conference.id)) && conference.registration_limit_exceeded?
                   Sorry, no places left
-            - if !current_user.nil? && current_user.tracks.where(program: conference.program).length > 0
-              = link_to "My Track Requests", conference_program_tracks_path(conference.short_title), class: 'btn btn-default'
+            - if current_user && current_user.tracks.where(program: conference.program).length > 0
+              = link_to 'My Track Requests', conference_program_tracks_path(conference.short_title), class: 'btn btn-default'
             - elsif can? :new, conference.program.tracks.new
-              = link_to "Submit Track Request", new_conference_program_track_path(conference.short_title), class: 'btn btn-default'
+              = link_to 'Submit Track Request', new_conference_program_track_path(conference.short_title), class: 'btn btn-default'
             - if !current_user.nil? && current_user.proposal_count(conference) > 0
               = link_to "My Proposals", conference_program_proposals_path(conference.short_title), class: 'btn btn-default'
             - elsif can? :new, conference.program.events.new