Add renderText to FeedStory for simple text repr of story

Summary:
This pushes the rendering of feed stories out of IRCDifferentialNotificationHandler and into feed.query

Also fixes bug in feed.query that broke attachment of related objects to stories.

Test Plan: echo '{"view": "text", "limit": 10}' | arc call-conduit feed.query

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, codeblock

Differential Revision: https://secure.phabricator.com/D4390