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