Provide context objects for remarkup mail rendering, fixing Phriction relative URIs in initial email

Summary:
Fixes T10969. Ref T13077. When you create a Phriction document with a relative link ([[ ./path/to/page ]]) the initial email currently points to the wrong place.

This is because the context object (the page) isn't passed to the markup engine. Without this context, the relative link is rendered as though it appeared somewhere else (like a task or revision) where relative links don't make sense.

Test Plan: Created a new Phriction document with a relative link to [[ ./porcupine_facts/starmap ]], saw a usable link in the resulting email.

Maniphest Tasks: T13077, T10969

Differential Revision: https://secure.phabricator.com/D19105