Hack up Asana story text so it's more useful

Summary: `FeedStory->renderText()` is garbage and I don't want to fix it in general until after T2222 / T2217. Provide an Asana-specific alternative for higher-quality feed stories (notably, including comment text).

Test Plan: {F51035}

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2852

Differential Revision: https://secure.phabricator.com/D6521