Add support for more generic notification

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.

website/site/content/docs/webapp/notifydueitems.md

@@ -1,6 +1,8 @@
 +++
 title = "Notify about due items"
 weight = 60
+draft = true
+render = false
 [extra]
 mktoc = true
 +++