Add docs, persist webhooks from gitops even when empty

docs/Configuration/yaml-files.md

@@ -565,6 +565,21 @@ For secrets, you can add [GitHub environment variables](https://docs.github.com/
 
 The `webhook_settings` section lets you define webhook settings for failing policy, vulnerability, and host status automations. Learn more about automations in Fleet [here](https://fleetdm.com/docs/using-fleet/automations).
 
+#### activities_webhook
+
+- `enable_activities_webhook` (default: `false`)
+- `destination_url` is the URL to `POST` to when an activity is generated (default: `""`)
+
+### Example
+
+```yaml
+org_settings:
+  webhook_settings:
+    activities_webhook:
+      enable_activities_webhook: true
+      destination_url: https://example.org/webhook_handler
+```
+
 #### failing_policies_webhook
 
 - `enable_failing_policies_webhook` (default: `false`)

server/service/appconfig.go

@@ -486,6 +486,12 @@ func (svc *Service) ModifyAppConfig(ctx context.Context, p []byte, applyOpts fle
 		}
 	}
 
+	// Webhook settings should always come from the new config,
+	// if there are no settings, disable webhooks. The unmarshal
+	// that merges the settings doesn't set the fields to the
+	// default values.
+	appConfig.WebhookSettings = newAppConfig.WebhookSettings
+
 	// If the license is Premium, we should always send usage statisics.
 	if license.IsPremium() {
 		appConfig.ServerSettings.EnableAnalytics = true