Center tardiness information

aleksis/apps/alsijil/frontend/components/coursebook/statistics/StatisticsTardinessCard.vue

 <template>
   <v-skeleton-loader v-if="loading" type="card" />
   <v-card v-else class="text-center">
-    <v-card-text>
+    <v-card-text class="d-flex flex-column align-center justify-center fill-height">
       <div class="text-h2">
         {{ tardinessCount }}
       </div>