[webinterface] add calendar settings to webinterface

internal/endpoints/configuration/configurationEndpoint.go

internal/endpoints/notification/calendar/calendar.go

internal/server/api.go

...ver/web/partials/settings-notifications-calendar.mustache

internal/server/web/partials/settings-notifications.mustache

internal/server/web/static/js/discovery.js

internal/server/web/static/js/settings-notifications.js