Wrap the feed text rendering stuff with `htmlspecialchars_decode`

Summary: I've only seen this be an issue with PhabricatorBot.

Test Plan: Comment on something with characters that are automatically converted like "<>", and see what the irc bot reports

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: epriestley, aran

Differential Revision: https://secure.phabricator.com/D8140