Summary: Implement Diffusion MetaMTA options. Also make the fake '{{config.option}}' rule work, and use Remarkup to render summaries as well as descriptions.
Test Plan: Looked at Diffusion rules, edited some, looked at setup issues, verified '{{config.option}}' linked to the right option.
Reviewers: codeblock, btrahan
Reviewed By: codeblock
CC: aran
Maniphest Tasks: T2255
Differential Revision: https://secure.phabricator.com/D4466
