Set up Cucumber for testing

This adds and configures Cucumber (using the cucumber-rails gem) in
preparation for adding feature tests.

As Frontend does not use a database, I have removed the DatabaseCleaner
configuration from `features/support/env.rb`.
Define specialist sector Cucumber feature
Request the singular form of the specialist sector tag type

As we've changed the content api to prefer singular tag types in its 
URLs, we should change these here too. These need to be consistently 
changed to make sure that the API adapter test stubs don't break.
Bump gds-api-adapters to 9.1.0

This lets us use the group_by parameter when requesting artefacts for a
tag from the Content API.
Display artefacts for specialist sectors in groups

Request the grouped form of artefacts for specialist sectors from the
Content API based on format, and display them in groups.

This includes some styling tweaks to the specialist sector list to fix
some positioning issues.
Merge pull request #529 from alphagov/specialist-sector-content-types

Group content in specialist navigation by its format