Make activity date formats consistent

with date formats elsewhere on the site. Everyone should be using the
render_datetime() helper and if they really want a different date
format, still user render_datetime() but pass their own date_format arg.

ckan/templates/_util.html

@@ -474,7 +474,7 @@
       </py:otherwise>
     </py:choose>
     <span class="date">
-      ${h.render_datetime(activity.timestamp, '%B %d %Y')}
+      ${h.render_datetime(activity.timestamp)}
     </span>
   </div>