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