Bumps gds-api-adapters gem version to 14.2.0
Adds services registration/initialization module

* Moves services registration/initialization out of ApplicationController
* Changes code to use Collections#services

Having the services initialized with the app caused changes to
BrowseControllerTest. This test previously stubbed Plek#website_root in
a before block, to ensure a consistent root for the URLs.

This PR maintains that behaviour by setting the GOVUK_WEBSITE_ROOT env var in the test helper, before
setting the Rails env.
Adds CollectionsApi service
Retrives curated content from the collections api

* Changes Subcategories to retrieve content from the collections api
Improves naming to be closer to agreed changes
Merge pull request #41 from alphagov/retrieve-curated-lists-from-collections-api

Retrieve curated lists from collections api

https://www.pivotaltracker.com/story/show/75806906