Add paging logic to get_a_service helper
Add paging logic to get_a_service helper

If we're looking for copyable services we can get into a situation where
there are no copyable services on the first page of results. This commit
adds paging logic copied from `get_brief_responses`.

alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests