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

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

app/models/integrations/app.rb

@@ -39,7 +39,8 @@ class Integrations::App
   def action
     case params[:id]
     when 'slack'
-      "#{params[:action]}&client_id=#{ENV.fetch('SLACK_CLIENT_ID', nil)}&redirect_uri=#{self.class.slack_integration_url}"
+      client_id = GlobalConfigService.load('SLACK_CLIENT_ID', nil)
+      "#{params[:action]}&client_id=#{client_id}&redirect_uri=#{self.class.slack_integration_url}"
     when 'linear'
       build_linear_action
     else
@@ -50,7 +51,7 @@ class Integrations::App
   def active?(account)
     case params[:id]
     when 'slack'
-      ENV['SLACK_CLIENT_SECRET'].present?
+      GlobalConfigService.load('SLACK_CLIENT_SECRET', nil).present?
     when 'linear'
       GlobalConfigService.load('LINEAR_CLIENT_ID', nil).present?
     when 'shopify'