R1:5caeb5c4db99

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}…