message: Add invisible placeholder for post slider controls
diff: <app-button class="-prev" v-if="page > 1" overlay trans @click.stop="goPrev">
  <app-jolticon icon="chevron-left" />
  </app-button>
  + <div v-else-if="post.media.length > 1" class="-prev" @click.stop></div>
  <app-button
  class="-next"
  >
  <app-jolticon icon="chevron-right" />
  </app-button>
  + <div v-else-if="post.media.length > 1" class="-next" @click.stop></div>
  </v-touch>
  <app-event-item-media-indicator
  