Wip email preferences

todo:
- form for setting up email preferences
- only send emails if user wants emails for that board
- rake task to make everyone want all emails
Simple require_user helper
Simple saving for email preferences
Only send people stuff they want

- Specifically, you get anything for a board you want all notifications
  for, and anything on a card you're assigned to OR are mentioned in
Bail unless a comment is passed
Save history before reloading
Wrangle trello api objects

- Weirdly the webhook sends just the card's short id whereas getting
  the card from the API doesn't (it just gives you the actual full short url)
Link to email preferences in the footer
Accept notifications for all boards for all users
Merge pull request #2 from a-warner/email_preferences

Email preferences