OpenAPI compliance: avoid $ref siblings (#1457)

This strives to fix all remaining cases where additional attributes
(most often 'description' but not only) are provided next to $ref
by wrapping $ref in allOf; and also drops allOf in a couple of places
where $ref is the only element under it.

data/api/client-server/definitions/event_batch.yaml

@@ -16,9 +16,7 @@ properties:
   events:
     description: List of events.
     items:
-      allOf:
-      - $ref: ../../../event-schemas/schema/core-event-schema/event.yaml
-      type: object
+      $ref: ../../../event-schemas/schema/core-event-schema/event.yaml
     type: array
 type: object
 title: EventBatch