Increase default activity page size to 20 (#2712)

If a group has a lot of activities at the same start time it
can cause not all of them to be returned.

Increasing the page size somewhat mitigates the effect but
it really needs to be resolved in the DRF CursorPagination.

src/activities/queries.js

@@ -106,7 +106,7 @@ export function useActivityListQuery ({
   feedbackPossible,
   hasFeedback,
   ordering,
-  pageSize = 10,
+  pageSize = 20,
 }, queryOptions = {}) {
   const query = useInfiniteQuery(
     queryKeyActivityList({ groupId, placeId, placeStatus, placeArchived, seriesId, activityTypeId, slots, feedbackPossible, hasFeedback, places, ordering, dateMin }),