MM-40813: Use config service more extensively (#19679)

https://mattermost.atlassian.net/browse/MM-40816

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/channels.go

@@ -247,9 +247,9 @@ func (ch *Channels) Stop() error {
 }
 
 func (ch *Channels) AddConfigListener(listener func(*model.Config, *model.Config)) string {
-	return ch.srv.AddConfigListener(listener)
+	return ch.cfgSvc.AddConfigListener(listener)
 }
 
 func (ch *Channels) RemoveConfigListener(id string) {
-	ch.srv.RemoveConfigListener(id)
+	ch.cfgSvc.RemoveConfigListener(id)
 }