Add UI for setting cancelled status

Add indications in event cards, event slot cards and the timetable for
an event or event slot being cancelled by striking it through and
dimming the text colour.  And a checkbox in the event and event slot
list to edit the cancelled status.  And a diff entry for the cancelled
status on events and event slots.

components/CardEventSlot.vue

@@ -3,7 +3,10 @@
 	SPDX-License-Identifier: AGPL-3.0-or-later
 -->
 <template>
-	<section class="eventSlot">
+	<section
+		class="eventSlot"
+		:class="{ cancelled: slot.cancelled || event?.cancelled }"
+	>
 		<hgroup>
 			<h3>{{ event?.name }}</h3>
 			<p>
@@ -75,6 +78,10 @@ function formatTime(time: DateTime) {
 	padding: 0.5rem;
 	border-radius: 0.5rem;
 }
+.eventSlot.cancelled>* {
+	text-decoration: line-through;
+	color: grey;
+}
 .eventSlot h3 {
 	margin: 0;
 }
@@ -82,7 +89,9 @@ function formatTime(time: DateTime) {
 	margin-block-start: 0.5rem;
 }
 
-.notice {
+.eventSlot .notice {
+	text-decoration: none;
+	color: CanvasText;
 	display: flex;
 	width: fit-content;
 	gap: 0.5rem;