Delegate methods to finder in ResultSetPresenter
Delegate methods to finder in ResultSetPresenter

By delegating these methods in one place rather than having them
throughout the class we make the interface we expect more explicit. That
should help us as we start refactoring the way these presenters hang
together.

Also rename `documents_noun` to `document_noun` for consistency.

alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend