Rename PhutilRemarkupRule subclasses

Summary: Ref T5655. Some discussion in D9839. Generally speaking, Phutil{$name}RemarkupRule is clearer than PhutilRemarkupRule{$name}.

Test Plan: Not yet tested... this is likely to break external code, so I'm not sure whether or not we want to pursue this.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D9993