Allow users to select QueryPanel search engines from a list
Summary: Ref T4986. Instead of requiring users to know the name of an application search engine class, let them select from a list.
Test Plan:
Created a new panel.
{F165468}
Reviewers: chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T4986
Differential Revision: https://secure.phabricator.com/D9500