Bump gds-api-adapters to 16.1.0

This adds support for the 'start' and 'count' parameters to the
Collections API adapter.
Pass 'start' and 'count' parameters to Subcategory

This changes the SubcategoriesController to pass the 'start' and 'count'
pagination parameters through to the Subcategory model when fetching a
sub-topic.

This adds some basic sanitisation of the parameters, forcing them to be
integers, and removes any blank parameters so that we can maintain 
defaults further upstream.

This also improves the test coverage around the latest_changes action of
the controller.