Consistent, concise styling of datables

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

app/views/admin/users/_event_registrations.html.haml

@@ -5,7 +5,7 @@
 
   .col-md-12
     .well
-      %table.table.table-bordered.table-striped.table-hover.datatable#event_registrations
+      %table.datatable#event_registrations
         %thead
           %th ID
           %th Conference

app/views/admin/users/_submissions.html.haml

@@ -4,7 +4,7 @@
       Submissions (#{@user.events.length}) for #{@user.name}
   .col-md-12
     .well
-      %table.table.table-bordered.table-striped.table-hover.datatable#submissions
+      %table.datatable#submissions
         %thead
           %th ID
           %th Conference

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

@@ -10,7 +10,7 @@
             = link_to 'Add User', new_admin_user_path, :class => 'button btn btn-default btn-info'
 .row
   .col-md-12.table-responsive
-    %table.table.table-striped.table-bordered.table-hover.datatable
+    %table.datatable
       %thead
         %th ID
         %th Confirmed?