MM-11931 Add support for AllowedUntrustedInternalConnections to be comma-separated (#11614)

* Add support for AllowedUntrustedInternalConnections to be comma-separated

* Add comprehensive test cases for fields splitting function

app/webhook_test.go

@@ -601,7 +601,7 @@ func TestTriggerOutGoingWebhookWithUsernameAndIconURL(t *testing.T) {
 	defer th.TearDown()
 
 	th.App.UpdateConfig(func(cfg *model.Config) {
-		*cfg.ServiceSettings.AllowedUntrustedInternalConnections = "localhost 127.0.0.1"
+		*cfg.ServiceSettings.AllowedUntrustedInternalConnections = "localhost,127.0.0.1"
 	})
 	createdPost := make(chan *model.Post)