Mass mandrill mailings

Still need a few admin pages before we push to hosted, but mass mailings fully working. Key features: 

  - Mandrill connection
  - Test that connection exists before showing mailings page
  - Allow default, custom, or no template
  - Markdown parsing of the message
  - Tracking opens, location, and clicks for campaigns
  - Campaigns tracked by subject+date
  - True verification (via API) that message has been sent
  - Forced "unsubscribe" link for templates that don't contain {{{unsubscribe_link}}} var

Yeah.