Manage notification channels separately and migrate

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.

modules/webapp/src/main/elm/Comp/ChannelForm.elm

@@ -48,17 +48,11 @@ type alias HttpModel =
     }
 
 
-type alias RefModel =
-    { channelType : ChannelType
-    }
-
-
 type Model
     = Matrix MatrixModel
     | Gotify GotifyModel
     | Mail MailModel
     | Http HttpModel
-    | Ref RefModel
 
 
 type Msg
@@ -147,11 +141,6 @@ initWith flags channel =
             , Cmd.none
             )
 
-        Data.NotificationChannel.Ref m ->
-            ( Ref { channelType = m.channelType }
-            , Cmd.none
-            )
-
 
 channelType : Model -> ChannelType
 channelType model =
@@ -168,9 +157,6 @@ channelType model =
         Http _ ->
             Data.ChannelType.Http
 
-        Ref ref ->
-            ref.channelType
-
 
 getChannel : Model -> Maybe NotificationChannel
 getChannel model =
@@ -187,9 +173,6 @@ getChannel model =
         Http mm ->
             Maybe.map Data.NotificationChannel.Http mm.value
 
-        Ref _ ->
-            Nothing
-
 
 
 --- Update
@@ -269,12 +252,3 @@ view texts settings model =
         Http m ->
             Html.map HttpMsg
                 (Comp.NotificationHttpForm.view texts.httpForm m.form)
-
-        -- Note: currently when retrieving hooks, this is not
-        -- send from the server. The server always sends
-        -- concrete channel details. However, it is possible
-        -- to create hooks with a reference to an existing
-        -- channel, but this is not supported in this client.
-        -- So this channel is ignored here.
-        Ref _ ->
-            span [ class "hidden" ] []