Refactor the notification script

Refactor the notification script

Reimplement most of the notification script logic. Create a separate
class for each notification type. Each class provides methods for
generating the list of recipients, the message subject, the message
body, the references to add at the end of the message and the message
headers. Additionally, a method for sending notification emails is
provided.

One major benefit of the new implementation is that both the generation
of recipients and message contents are much more flexible. For example,
it is now easily possible to make user-specific adjustments to every
single notification of a batch.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>