Consistent, concise styling of datables

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

app/views/admin/roles/_users.html.haml

@@ -1,7 +1,7 @@
 .page-header
   %h3 Users (#{users.length})
 - if users.present?
-  %table.table.table-striped.table-bordered.table-hover.datatable#users
+  %table.datatable#users
     %thead
       - if ( can? :toggle_user, @role )
         %th.col-md-1