Use remarkup rule priorities in Phabricator

Summary:
Depends on D6329. This fixes http://www.example.com/D123, which currently gets the "D123" rendered, after addition of the Asana rule. It also removes a hack for object refernces.

Basically, the "hyperlink" rule needs to happen after rules which specialize hyperlinks (Youtube, Asana) but before rules which apply to general text (like the Differential and Maniphest rules). Allow these rules to specify that they have higher or lower priority.

Test Plan: Asana rules, Differential rules and Diffusion rules now all markup correctly.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6330