Add basic functionality to add own custom events

aleksis/core/frontend/components/app/whoAmI.graphql

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

...nd/components/calendar/custom_event/CustomEventDialog.vue

...tend/components/calendar/custom_event/customEvent.graphql

...ntend/components/calendar_feeds/details/CustomDetails.vue

...frontend/components/notifications/myNotifications.graphql

aleksis/core/frontend/messages/de.json

aleksis/core/frontend/messages/en.json

...s/core/migrations/0056_alter_globalpermissions_options.py

aleksis/core/migrations/0057_customevent_location.py

aleksis/core/models.py

aleksis/core/schema/__init__.py

aleksis/core/schema/custom_event.py