Feed - fix some whacky "text mode" rendering code

Summary: ...add a "renderingTarget" to FeedStory and use it as appropos. Overall, not a ton of changes was necessary to make this work. I think this could be made to be even cleaner by going through each and every feed story and re-implementing as necessary with the full toolset available. But this is good enough for now I think, and just something to keep cleaning up when we're in here. Fixes T4630.

Test Plan: made a task. gave it a token. viewed my feed - saw stories. used conduit.feed.query with mode == 'text' and got good readable results.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: spicyj, epriestley, Korvin

Maniphest Tasks: T4630

Differential Revision: https://secure.phabricator.com/D8750