Make repository symbol references work with DocumentEngine

Summary: Ref T13105. Ref T13047. This makes symbol indexes work with DocumentEngine in Files, and restores support in Diffusion.

Test Plan: Command-clicked stuff, got taken to the symbol index with reasonable metadata in Diffusion, Differential and Files.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105, T13047

Differential Revision: https://secure.phabricator.com/D19307