Allow Subcategory to specify start and count params

This changes the Subcategory model to allow the "start" and "count"
parameters to be provided as arguments to the `.find` method. When
present, these arguments are included into the request to Collections
API, which in turn passes these arguments to Rummager.
Bump gds-api-adapters to 16.2.0

This adds support for the 'start' and 'count' parameters to the
Collections API adapter and test helper.
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.
Explicitly reset services between tests

This adds a new test support script which resets the Collections API
service between tests. This is so that, when we override the Collections
API with a stub or mock, the mocks do not leak into other tests which
rely on the standard service behaviour.
Expose 'start' and 'total' values from API response

This changes the Subcategory model to expose the 'documents_total' and
'documents_start' keys from the Collections API response as instance
methods.
Add a ChangedDocumentsPaginationPresenter

This adds a new presenter which encapsulates behaviour to determine
whether a paginated list of changed documents has next or previous pages,
their starting values, page numbers and paths.
Display pagination on the latest changes view

This changes the latest changes view template to display previous and
next page links. The view uses the ChangedDocumentsPaginationPresenter
to infer whether a next or previous page link should be displayed.

The markup is identical to the pagination links used for search results
in Frontend, and I've copied the stylesheet and arrow sprite image over
from there too.

This also includes a full integration test for the pagination, which
stubs the API requests using the Collections API adapter's test helper.
Merge pull request #59 from alphagov/add-pagination-to-latest-feed

Add pagination to the latest changes feed