Show a message when the calendar has no events.

Closes #34. Upcoming and Past tabs explain an empty list instead of leaving the content area blank.

schasite/templates/schasite/calendar2.html

@@ -136,6 +136,12 @@
                         </div>
                     </div>
                     
+                    {% empty %}
+                    <div class="col-12">
+                        <div class="alert alert-light border text-center mb-0" role="status">
+                            There are no upcoming events to display.
+                        </div>
+                    </div>
                     {% endfor %}
                 </div>
             </div>
@@ -179,9 +185,13 @@
                                 </div>
                             </div>
                         </div>
+                    {% empty %}
+                    <div class="col-12">
+                        <div class="alert alert-light border text-center mb-0" role="status">
+                            There are no past events to display.
+                        </div>
+                    </div>
                     {% endfor %}
-                     
-                    
                 </div>
             </div>
         </div>