Use action list and property list elements in Ponder

Summary: This is sort of silly as-is, but automatically exposes flagging and will give subscribe/unsubscribe and "Subscribers" a place to plug into shortly. For context, see D3637 and T1808.

Test Plan: {F20550}

Reviewers: pieter, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1808

Differential Revision: https://secure.phabricator.com/D3641