Merge branch 'master' into 211-implement-vue-substitution-frontend

...s/apps/chronos/frontend/components/LessonEventSubject.vue

aleksis/apps/chronos/frontend/components/SelectTimetable.vue

...ntend/components/calendar_feeds/details/LessonDetails.vue

.../components/calendar_feeds/details/SupervisionDetails.vue

...nd/components/calendar_feeds/event_bar/LessonEventBar.vue

...mponents/calendar_feeds/event_bar/SupervisionEventBar.vue

aleksis/apps/chronos/frontend/index.js

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

aleksis/apps/chronos/models.py

aleksis/apps/chronos/preferences.py