Style feed stories with phabricator-remarkup

Summary: Fixes T6316. Wraps feed content in remarkup.

Test Plan: Post a comment with a quote on Task, go to Feed and see the quote properly styled.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6316

Differential Revision: https://secure.phabricator.com/D10788