Merge branch 'subscription' Pull Request #125

* subscription: (25 commits)
  link directly to subscription management in mails
  only use mailfromnobody for bulk mails
  added missing context for list mails
  readded mailfromnobody to subscription sending
  correctly escape diffs in HTML mails
  fixed lists in HTML mails
  simplified subscription->add() code a bit
  comment adjusted
  removed unused vars
  removed data parameter in subscription_handle_post()
  fixed tests
  some reformatting
  added compatibility function
  moved registration notification to subscription class
  fixed merge error in inc/auth.php
  consolidate more notification code in subscription class
  minor cleanup
  initialize new subscriptions with current time
  fixed subscription management
  correctly check if subscriptions are enabled
  ...