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