Allow Nuance source definitions to add actions to source views
Summary:
Ref T8783. If you have a source (like a "report bug" form), let it put a link (like "View Form") on the source detail page.
This also straightens out getting definitions from sources, which had a bug with the modern way we do `PhutilClassMapQuery`.
Specifically, if you called the old mechanism on two different sources, they'd return the same definition object, but they need to return different definitions.
Test Plan:
{F747093}
{F747092}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T8783
Differential Revision: https://secure.phabricator.com/D13966