R1:f55c082e65f9

Publish Differential stories into feed

Summary: Basic hookup for Differential -> Feed. Also introduces "one-line"
stories for less-important stuff.
Test Plan: Interacted with some revisions, got feed stories out of it.
Reviewed By: jungejason
Reviewers: jungejason, aran, tuomaspelkonen, codeblock
CC: aran, jungejason
Differential Revision: 632