UI/UX changes related to tracks

Add confirmed and cfp_active scopes to Track
Render markdown in track's description
Make the views more similar to the proposal/events views
Change the sequence of columns in admin/Tracks#index
Remove the short_name column from the index views
Add button "My Tracks" in the user menu
Fix track count in proposals
Add details of track in admin/Tracks#show
Use tabs to show track details and events

app/views/proposals/_encouragement_text.html.haml

@@ -4,7 +4,7 @@
     = "#{event_types(@conference)}."
   - if @program.tracks.any?
     Proposals should fit in one of the
-    = "#{pluralize(@program.tracks.count, 'track')}:"
+    = "#{pluralize(@program.tracks.confirmed.cfp_active.count, 'track')}:"
     = "#{tracks(@conference)}."
   - if @program.cfp_open?
     The submission period has begun