It's more convenient to manage notification channels separately, as it
is done with email settings. Notification hook and other forms are
adopted to only select channels. Hooks can now use more than one
channel.
This is a start to have different kinds of notifications. It is
possible to be notified via e-mail, matrix or gotify. It also extends
the current "periodic query" for due items by allowing notification
over different channels. A "generic periodic query" variant is added
as well.
