Consistent, concise styling of datables

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

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

@@ -8,7 +8,7 @@
 - if @conference.tickets.any?
   .row
     .col-md-12
-      %table.table.table-hover.datatable#tickets
+      %table.datatable#tickets
         %thead
           %th Title
           %th Price