R1:ece9d792b223

Build basic infrastructure for an activity feed

Summary: This defines an extremely basic version of an activity feed, like
Facebook's news feed. It doesn't do much of interest yet.
Test Plan: Published some feed stories:
https://secure.phabricator.com/file/view/PHID-FILE-5061aa72105bbdc05b21/
Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
Commenters: codeblock, jungejason
CC: aran, epriestley, codeblock, tuomaspelkonen, jungejason
Differential Revision: 593