R1:163f2c4262a2

Refine available filters and defaults for relationship selection

Summary:
Ref T4788. Fixes T10703.

In the longer term I want to put this on top of ApplicationSearch, but that's somewhat complex and we're at a fairly good point to pause this feature for feedback.

Inch toward that instead: provide more appropriate filters and defaults without rebuilding the underlying engine. Specifically:

- No "assigned" for commits (barely makes sense).
- No "assigned" for mocks (does not make sense).
- Default…