Consistent, concise styling of datables

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

app/views/admin/events/show.html.haml

@@ -18,7 +18,7 @@
       = render 'proposal'
     #history-content.tab-pane
       .col-md-12
-        %table.table.table-striped.table-bordered.table-hover.datatable
+        %table.datatable
           %thead
             %th ID
             %th Description