Put a sigil on remarkip-code-block in PhutilRemarkupEngineRemarkupCodeBlockRule

Summary: Also modified few test cases.

Test Plan: I made the necessary changes, differential is loading diffs as usual. Let me know if I have done it correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, AnhNhan

Maniphest Tasks: T3007

Differential Revision: https://secure.phabricator.com/D5888