Add autocomplete for Phriction documents on "[[ ..." in Remarkup

Summary: Depends on D19107. Ref T13077. The underlying datasource may need some adjustment but this appears to work properly locally.

Test Plan: Typed [[ por locally, was suggested "Porcupine Facts". Typed [[ / ]], saw it render as a reference to the wiki root instead of the install root.

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19108