Add loading placeholders

Not shure if they should be kept insided the main
StatisticsForPersonCard or pushed into the subcomponents.

Previously there was a statistics undefined error while apollo was
still loading.

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

 <template>
   <v-card>
-    <v-card-title v-if="compact">
+    <v-skeleton-loader v-if="$apollo.loading"
+      type="card-heading"
+    />
+    <v-card-title v-else-if="compact">
       {{ $t("alsijil.coursebook.statistics.person_compact.title") }}
       <v-spacer />
       <!-- TODO: Maximize button -->
@@ -8,19 +11,30 @@
     <v-card-title v-else>
       {{ $t("alsijil.coursebook.statistics.person_page.title") }}
     </v-card-title>
-    <statistics-absences-card
+
+    <v-skeleton-loader v-if="$apollo.loading"
+      type="card"
+    />
+    <statistics-absences-card v-else
       :absence-reasons="statistics.absenceReasons"
     />
-    <statistics-tardiness-card
+    <v-skeleton-loader v-if="$apollo.loading"
+      type="card"
+    />
+    <statistics-tardiness-card v-else
       :tardiness-sum="statistics.tardinessSum"
       :tardiness-count="statistics.tardinessCount"
     />
-    <statistics-extra-marks-card
+    <v-skeleton-loader v-if="$apollo.loading"
+      type="card"
+    />
+    <statistics-extra-marks-card v-else
       :extra-marks="statistics.extraMarks"
     />
-    <statistics-personal-notes-list
-      v-if="compact"
+    <v-skeleton-loader v-if="compact && $apollo.loading"
+      type="card"
     />
+    <statistics-personal-notes-list v-else-if="compact" />
   </v-card>
 </template>