Expose Drydock authorizations via Conduit

Summary:
DrydockAuthorizationSearchEngine was being used solely to display authorizations for a specific blueprint from the web UI and consequently expected that callers set a specific blueprint before performing a query. Here we check to see if a blueprint has been set in cases where the engine could be operating from either Conduit or the web.

Ref T11694

Test Plan:

• called the API method from the console
• approved an authorization
• followed the "view all" link from a blueprint page

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T11694

Differential Revision: https://secure.phabricator.com/D16592