add broadcast preview component

src/components/model/BroadcastPreview.vue

+<template>
+  <app-card :title="broadcast.subtitle || series.name" :description="description" :image="coverImage">
+    <template v-slot:footer>
+      <div class="f-1 opaque-7 mt-1">
+        <app-time :date="new Date(broadcast.start_time)" use-exact
+                  :aria-label="$t('broadcast_preview.start_time')"/>
+      </div>
+    </template>
+  </app-card>
+</template>
+
+<script>
+  import { imageUrl } from './mixins'
+
+  export default {
+    mixins: [
+      imageUrl('series', 'cover_image', 'seriesCoverImage'),
+      imageUrl('broadcast', 'cover_image', 'broadcastCoverImage')
+    ],
+    props: {
+      broadcast: Object,
+      series: Object
+    },
+    computed: {
+      coverImage () {
+        return {
+          src: this.broadcastCoverImage || this.seriesCoverImage,
+          size: 72,
+          autoSize: true,
+          aspectRatio: 1,
+          contain: true
+        }
+      },
+      description () {
+        return this.broadcast.description || this.$t('broadcast_preview.no_description')
+      }
+    },
+    inject: ['dataSources']
+  }
+</script>
+
+<i18n lang="yaml">
+  en:
+    broadcast_preview.no_description: No description available.
+    broadcast_preview.start_time: Start of program
+
+  de:
+    broadcast_preview.no_description: Keine Beschreibung verfügbar.
+    broadcast_preview.start_time: Sendungsbeginn
+</i18n>