Consistent, concise styling of datables

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

app/views/admin/reports/_events_with_requirements.html.haml

@@ -7,7 +7,7 @@
       %p.text-muted
         All submissions where the speakers have special requirements
 .col-md-12
-  %table.table.table-striped.table-bordered.table-hover.datatable
+  %table.datatable
     %thead
       %th ID
       %th Title