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.

modules/webapp/src/main/elm/Main.elm

@@ -14,6 +14,7 @@ import App.View2
 import Browser exposing (Document)
 import Browser.Navigation exposing (Key)
 import Data.Flags exposing (Flags)
+import Data.NotificationChannel
 import Data.UiSettings
 import Html exposing (..)
 import Html.Attributes exposing (..)