Alerting: Fix issue with Slack contact point validation (#47559)

* secureFields and secureSettings

* revert channelIndex

* readd lost code

* use specific return

* register secure fields and use not hard coded index

* fix for determineReadOnly

* fix lint error

* fix test suite

Co-authored-by: gillesdemey <gilles.de.mey@gmail.com>

pkg/services/ngalert/notifier/available_channels.go

@@ -383,7 +383,7 @@ func GetAvailableNotifiers() []*alerting.NotifierPlugin {
 					Description:  "Specify channel, private group, or IM channel (can be an encoded ID or a name) - required unless you provide a webhook",
 					PropertyName: "recipient",
 					Required:     true,
-					DependsOn:    "secureSettings.url",
+					DependsOn:    "url",
 				},
 				// Logically, this field should be required when not using a webhook, since the Slack API needs a token.
 				// However, since the UI doesn't allow to say that a field is required or not depending on another field,
@@ -397,7 +397,7 @@ func GetAvailableNotifiers() []*alerting.NotifierPlugin {
 					PropertyName: "token",
 					Secure:       true,
 					Required:     true,
-					DependsOn:    "secureSettings.url",
+					DependsOn:    "url",
 				},
 				{
 					Label:        "Username",
@@ -463,7 +463,7 @@ func GetAvailableNotifiers() []*alerting.NotifierPlugin {
 					PropertyName: "url",
 					Secure:       true,
 					Required:     true,
-					DependsOn:    "secureSettings.token",
+					DependsOn:    "token",
 				},
 				{ // New in 8.4.
 					Label:        "Endpoint URL",