Change default_page_size to Kaminari default
Change default_page_size to Kaminari default

The default `kaminari` page size is 25, but we were internally using 50 when fetching data before passing into `kaminari` for pagination. This resulted in some results not being displayed because we were paging in increments of 50 while `kaminari` was paging in increments of 25.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall