Only show visible groups in doc collection contents

Groups containing no documents or no published documents would be
displayed in the contents list but not in the main content.
Merge pull request #1087 from alphagov/only-show-contents-for-non-empty-collection-groups

Only show visible groups in doc collection contents

Groups containing no documents or no published documents would be
displayed in the contents list but not in the main content.

Follow up for https://www.pivotaltracker.com/story/show/61211960