Allow repository manager to see embargoed content

Edited app/helpers/hydramata/solr_helper.rb to not add embargo to
the solr query if the current user is a repository manager.

Added a new test to verify that a repository manager can discover
another user's embargoed work.

Note: Managers could already see the show pages for embargoed
works.  This sub-task only needed to fix discoverability.

HYDRASIR-355 #close
Merge pull request #487 from projecthydra/feature/HYDRASIR-355

Allow repository manager to see embargoed content