WIP: Calendar feeds in frontend

...is/core/frontend/components/calendar/CalendarOverview.vue

aleksis/core/frontend/components/calendar/CalendarSelect.vue

...ore/frontend/components/calendar/calendarOverview.graphql

aleksis/core/frontend/messages/en.json

aleksis/core/frontend/routes.js

aleksis/core/mixins.py

aleksis/core/schema/__init__.py

aleksis/core/schema/calendar.py

aleksis/core/util/core_helpers.py

pyproject.toml