R1:8daaf5ef2145

Pass a real context object to Phriction previews, fixing mentions

Summary:
Fixes T13662. Phriction currently passes a map as a "context object", but this code is ancient and predates the modern meaning of a "context object". In modern code, context objects should be real objects.

Provide a real object as a context object. We do this by either loading the actual document or constructing a synthetic version of it.

Test Plan:
- Edited an existing document, observing the preview:
- Used a mention rule…