model: move notification types from Notification to NotificationModel

This commit is part of the removal of the UI notification feature from
Kallithea, which is not deemed useful in its current form. Only email
notifications are preserved.

As there is no database storage of notifications anymore, the Notification
class will be removed. However, the notification type definitions are still
used for email notifications, and need to live somewhere. As creating
notifications is always passing via NotificationModel, it makes sense to
move the types there.