set minimum date for datepicker and fullcalendar instances to 1970-01-01, re #2076

Merge request studip/studip!1906

resources/assets/javascripts/lib/fullcalendar.js

@@ -355,6 +355,9 @@ class Fullcalendar
             },
             minTime: '08:00:00',
             maxTime: '20:00:00',
+            validRange: {
+                start: '1970-01-01'
+            },
             height: 'auto',
             contentHeight: 'auto',
             firstDay: 1,

resources/assets/javascripts/studip-ui.js

@@ -578,7 +578,8 @@ import eventBus from "./lib/event-bus.ts";
             if (value !== instance.lastVal) {
                 $(this).change();
             }
-        }
+        },
+        minDate: new Date(1970, 0)
     };
 
     $.datepicker.setDefaults({