API views using the CustomCursorPagination have a with_count parameter that does not work

Found while working on #1116 (closed): The CustomCursorPagination class is similar to the original DRF CursorPagination class but includes an extra with_count=[true/false] query parameter to optionally still add a count property to the response. However, with_count currently requires the arkindex.project.mixins.CursorPaginationViewMixin to be used on the view class, and the requirement is undocumented. A view that uses CustomCursorPagination without the mixin has with_count available on OpenAPI, but it will have no effect.

ListProcessElements is the only endpoint that correctly uses the mixin with the pagination class. ListCorpusWorkerVersions uses the pagination class but without the mixin, causing with_count to be present without actually working.

It is possible to rewrite the pagination class to avoid the mixin entirely, but it might be simpler to have it be in a separate class, where we only enable with_count when it is seriously required, as the whole point of cursor pagination is to be performant. A count simply cannot be performant.