Finder beta status in details hash

As more Orgs start using Finders, we want to show to users that the
Finder and it's content are still in Beta.

This commit adds a beta flag to the metadata and is sent as part of the
details hash in the Finder when it's published to the publishing API.
Remove duplicate and re-add details hash to ContentItemPresenter
Merge pull request #81 from alphagov/finder-beta-status-in-details-hash

Finder beta status in details hash