Support `M123/456` mock references in Remarkup

Summary: Fixes T4729. This form is a little fluff, but we show it in the URI when you click an anchor on the page, and doing so seems desirable. I think it's reasonable to support this form, given that it appears in the URI.

Test Plan: Wrote some stuff like `M60`, `M60/71`, `M60/72/`, `M60/73/#13` and saw it all get picked up and rendered/linked properly.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4729

Differential Revision: https://secure.phabricator.com/D9555