Support relative links in Phriction

Summary:
Resolves T7691. This turned out more complex than I really wanted, mainly because I needed to feed the slug information through to both the document renderer and the preview window that appears in the edit controller.

After this change, you can now create relative links in Phriction by doing [[ ./../some/relative/path ]]. Relative paths aren't handled anywhere else (they'll still render, but the dots are turned into a literal 'dot' as per existing behaviour).

Test Plan: Created some Phriction documents with relative links, saw them all link correctly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T7691

Differential Revision: https://secure.phabricator.com/D15732