Tweak proceedings details.

proceedings/templates/partials/proceedings/description.html

 <h3>{{ proceedings.event_name }} {% if proceedings.event_suffix %}({{ proceedings.event_suffix }}){% endif %}</h3>
 <div class="mb-4">
-  This event was held from {{ proceedings.event_start_date }} to {{ proceedings.event_end_date }}.
+  Event dates: from {{ proceedings.event_start_date }} to {{ proceedings.event_end_date }}.
   <div class="my-1">
     <strong>Submission deadline: {{ proceedings.submissions_deadline }}</strong>.
   </div>