added second button to compare wiki revisions above the list

src/plainui/jinja2/plainui/static_page_history.html.j2

@@ -28,6 +28,7 @@
 
       <p>{{ _("Wiki__history__rev_help") }}</p>
 
+      <input type="submit" class="btn btn-primary m-2" value="{{ _('Compare') }}">
       <ul>
         {% for history_entry in history %}
           <li class="list-unstyled">