Beta label for certain Finders

As we launch Finders which may be considered in Beta, we want a way to
display this on a per finder basis. We set this in the metadata in
Finder API, which gets sent the content store as part of the details
hash.

This commit sets this on the Finder object and displays the beta label
if this is true.
Merge pull request #108 from alphagov/beta-label-for-certain-finders

Beta label for certain Finders