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

spec/features/tracks_spec.rb

@@ -39,7 +39,7 @@ feature Track do
       within('table#tracks') do
         expect(page.has_content?(track.name)).to be false
         expect(page.has_content?(track.description)).to be false
-        expect(page.assert_selector('tr', count: 1)).to be true
+        expect(page.has_content?('No data available in table')).to eq true
       end
     end