Help tablesorter see 'Month Year' dates as dates with a hidden day digit. Fixes issue #2921.

- Legacy-Id: 17463

ietf/templates/group/milestones.html

@@ -25,7 +25,8 @@
   	    <span class="label label-success">{{ milestone.resolved }}</span>
   	  {% else %}
         {% if group.uses_milestone_dates %}
-  	      {{ milestone.due|date:"M Y" }}
+           <!-- The first hidden digit is to help tablesorter see this as a date -->
+           <span style="display:none">1</span> {{ milestone.due|date:"M Y" }} 
         {% else %}
           {% if forloop.first %}Last{% endif %}
           {% if forloop.last %}Next{% endif %}