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, saw a preview.
  • Used [[ a ]] and [[ ./a ]] absolute and relative reference rules, saw accurate previews.
• Edited a new document, observing the preview:
  • Used a mention rule, saw a preview.
  • Used absolute/relative references, saw accurate previews.
• Grepped for other references to the removed properties (phriction.isPreview and phriction.slug), found none remaining.

Reviewers: 0

Reviewed By: 0

Maniphest Tasks: T13662

Differential Revision: https://secure.phabricator.com/D21709