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/Messages/Comp/ChannelForm.elm

@@ -0,0 +1,47 @@
+{-
+   Copyright 2020 Eike K. & Contributors
+
+   SPDX-License-Identifier: AGPL-3.0-or-later
+-}
+
+
+module Messages.Comp.ChannelForm exposing
+    ( Texts
+    , de
+    , gb
+    )
+
+import Messages.Basics
+import Messages.Comp.NotificationGotifyForm
+import Messages.Comp.NotificationHttpForm
+import Messages.Comp.NotificationMailForm
+import Messages.Comp.NotificationMatrixForm
+
+
+type alias Texts =
+    { basics : Messages.Basics.Texts
+    , matrixForm : Messages.Comp.NotificationMatrixForm.Texts
+    , gotifyForm : Messages.Comp.NotificationGotifyForm.Texts
+    , mailForm : Messages.Comp.NotificationMailForm.Texts
+    , httpForm : Messages.Comp.NotificationHttpForm.Texts
+    }
+
+
+gb : Texts
+gb =
+    { basics = Messages.Basics.gb
+    , matrixForm = Messages.Comp.NotificationMatrixForm.gb
+    , gotifyForm = Messages.Comp.NotificationGotifyForm.gb
+    , mailForm = Messages.Comp.NotificationMailForm.gb
+    , httpForm = Messages.Comp.NotificationHttpForm.gb
+    }
+
+
+de : Texts
+de =
+    { basics = Messages.Basics.de
+    , matrixForm = Messages.Comp.NotificationMatrixForm.de
+    , gotifyForm = Messages.Comp.NotificationGotifyForm.de
+    , mailForm = Messages.Comp.NotificationMailForm.de
+    , httpForm = Messages.Comp.NotificationHttpForm.de
+    }