add broadcast preview component

<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>