Refactor calendar overview in more different components

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

...rontend/components/calendar/CalendarDownloadAllButton.vue

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

...ore/frontend/components/calendar/CalendarWithControls.vue

aleksis/core/frontend/components/calendar/calendar.graphql

...s/core/frontend/components/calendar/calendarFeeds.graphql

aleksis/core/frontend/components/calendar/calendarMixin.js

...rontend/components/calendar/calendarSelectedFeedsMixin.js

aleksis/core/mixins.py

aleksis/core/models.py

aleksis/core/schema/calendar.py