Consistent, concise styling of datables

Add a semantic class to aggregate bootstrap classes and consistently
applyt them to all datatables.

app/views/admin/tracks/index.html.haml

@@ -33,7 +33,7 @@
 .row
   .col-md-12
     - if @tracks.any?
-      %table.table.table-hover.table-striped.table-bordered.datatable#tracks
+      %table.datatable#tracks
         %thead
           %th ID
           %th Name