Merge branch origin/266-add-statistics-page-for-absences

.../frontend/components/coursebook/statistics/MockPerson.vue

...omponents/coursebook/statistics/StatisticsForGroupTab.vue

...ponents/coursebook/statistics/StatisticsForPersonCard.vue

...ponents/coursebook/statistics/StatisticsForPersonPage.vue

...ponents/coursebook/statistics/StatisticsTardinessCard.vue

...ntend/components/coursebook/statistics/statistics.graphql

aleksis/apps/alsijil/frontend/index.js

aleksis/apps/alsijil/frontend/messages/de.json

aleksis/apps/alsijil/frontend/messages/en.json

aleksis/apps/alsijil/schema/__init__.py

aleksis/apps/alsijil/schema/statistics.py