Default remarkup code blocks to plain text, not PHP

Summary:
Ref T6823. If we have no language information for a block, render it without syntax highlighting instead of with PHP syntax highlighting.

Thinking is to try this, see who notices/complains, and then pursue more tailored defaults if they seem worthwhile.

Test Plan: Wrote a code block with text in it, saw no default highlighting.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6823

Differential Revision: https://secure.phabricator.com/D11091