feat: move Slack config to installation settings (#11548)

- enable Slack Configuration via InstallationConfig
- list Slack integration in super admin settings

lib/global_config_service.rb

@@ -1,6 +1,6 @@
 class GlobalConfigService
   def self.load(config_key, default_value)
-    config = ENV.fetch(config_key) { GlobalConfig.get(config_key)[config_key] }
+    config = GlobalConfig.get(config_key)[config_key]
     return config if config.present?
 
     # To support migrating existing instance relying on env variables

lib/integrations/slack/hook_builder.rb

@@ -32,8 +32,8 @@ class Integrations::Slack::HookBuilder
   def fetch_access_token
     client = Slack::Web::Client.new
     slack_access = client.oauth_v2_access(
-      client_id: ENV.fetch('SLACK_CLIENT_ID', 'TEST_CLIENT_ID'),
-      client_secret: ENV.fetch('SLACK_CLIENT_SECRET', 'TEST_CLIENT_SECRET'),
+      client_id: GlobalConfigService.load('SLACK_CLIENT_ID', 'TEST_CLIENT_ID'),
+      client_secret: GlobalConfigService.load('SLACK_CLIENT_SECRET', 'TEST_CLIENT_SECRET'),
       code: params[:code],
       redirect_uri: Integrations::App.slack_integration_url
     )