Fix over-escaping in text-mode rendering in Remarkup

Summary:
Fixes T2684. Currently, we use markup rules to restore text replacements, even in text mode. Instead, just do it normally when operating in text mode.

This fixes double-escaping in some email and, e.g., the Asana stories published by D6521.

Test Plan: Rendered remarkup with ' characters in text mode with replacements; saw ' in output instead of HTML entities.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2684

Differential Revision: https://secure.phabricator.com/D6522