Email maintainers when new version released

Similar to #230.

This is a good way to be notified that releasing a new version actually worked. This is especially useful when publishing packages using an automated process. Also it could be helpful as a way to notice unwanted releases, for example if an account was compromised.

Other package repositories that I use, including npm and RubyGems, have features like this.

Read issue #230 and trace the package publishing and automated release flow in hackage-server. The issue does not name files, tests, recipients, or configuration, so those requirements need clarification before implementation. Done should include maintainer notification after a successful release and a way to notice unwanted releases.

Written by the indexing model from the issue text.