Allow Phriction links to link to non-Phriction URIs
Summary: If the link text is a URI, just treat it as a nameable (and possibly
relative) URI link. See tasks.
Test Plan: Copy/pasted the doc example into Phriction, links worked.
Reviewers: skrul, hunterbridges, jungejason, tuomaspelkonen, aran
Reviewed By: jungejason
CC: aran, jungejason
Differential Revision: 882