Merge branch 'master' into absenzen

# Conflicts:
#	aleksis/apps/alsijil/frontend/components/coursebook/Coursebook.vue
#	aleksis/apps/alsijil/frontend/components/coursebook/CoursebookDay.vue
#	aleksis/apps/alsijil/frontend/components/coursebook/absences/ManageStudentsDialog.vue
#	aleksis/apps/alsijil/frontend/messages/en.json
#	aleksis/apps/alsijil/models.py
#	aleksis/apps/alsijil/schema/__init__.py