Allow action list items to render as forms

Summary: In some cases, we want an action item (like "Subscribe") to effect a write that needs a CSRF check. Allow such items to render as forms so they gracefully degrade if JS is FUBAR'd. D3499 has a specific example.

Test Plan: Loaded new UI example page, clicked all the actions.

Reviewers: btrahan, vrana, avivey

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3596