Name coursebook query result items

The queryMixin assumes query results are named items.
It's already customary in other queries.
Dynamically detecting and adapting the item name is not possible.

aleksis/apps/alsijil/frontend/components/coursebook.graphql

 query gqlDocumentationsForCoursebook($courseId: ID!, $dateStart: Date!, $dateEnd: Date!) {
-  documentationsForCoursebook(courseId: $courseId, dateStart: $dateStart, dateEnd: $dateEnd) {
+  items: documentationsForCoursebook(courseId: $courseId, dateStart: $dateStart, dateEnd: $dateEnd) {
     id
     course {
       id