Pageable default page size

In 7f3aa83e I limited the max page size for Subset and Dataset lists, but that was not really my intention: I wanted to set the default page size if l parameter is not specified by the user.

This piece of code Integer.min(l == null ? maxPageSize : l, maxPageSize) in Pagination needs to be updated to support a new defaultPageSize parameter:

Integer.min(l == null ? defaultPageSize : l, maxPageSize)

In Subset and Dataset API controllers, the defaultPageSize should be 20 and maxPageSize set at 50.