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:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8783

Differential Revision: https://secure.phabricator.com/D13966