Provide a public view of feed

Summary: Depends on D628. Provides a config option so you can set up a public
feed, which you can iframe. This needs some work but sort of works.
Test Plan: Loaded the public feed as a logged-out user.
Reviewed By: codeblock
Reviewers: jungejason, tuomaspelkonen, aran, codeblock
CC: aran, codeblock
Differential Revision: 635