Implement EventID to scope ClientIDs and Entry IDs

Implement an EventID saved in settings. Currently this is used to scope
clientIDs and entryIDs to an event. The client checks the event currently going on on
the server, and discards its localstorage (containing the clientID) if
it has changed

backend/templates/base.html

@@ -108,7 +108,7 @@
     {% block extrajs %}{% endblock %}
     <script>
         $(document).ready(function () {
-            loadOrGenerateClientId()
+            checkEventID()
             // get current URL path and assign 'active' class
             var pathname = window.location.pathname;
             $('.navbar-nav > li > a[href="' + pathname + '"]').parent().addClass('active');
@@ -130,6 +130,23 @@
                 localStorage.setItem("clientId", create_UUID())
             }
         }
+
+        function getClientId() {
+            return localStorage.getItem("clientId")
+        }
+
+        async function checkEventID() {
+            const localEventID = localStorage.getItem("eventID")
+            const resp = await fetch("/api/events/current")
+            const respJson = await resp.json()
+            const remoteEventID = respJson.event
+
+            if (localEventID == null || localEventID != remoteEventID) {
+                localStorage.clear()
+                localStorage.setItem("eventID", remoteEventID)
+                loadOrGenerateClientId()
+            }
+        }
     </script>
 </body>