feat: notify on slack disconnect (#1323)

app/mailers/administrator_notifications/channel_notifications_mailer.rb

@@ -0,0 +1,10 @@
+class AdministratorNotifications::ChannelNotificationsMailer < ApplicationMailer
+  def slack_disconnect(account)
+    return unless smtp_config_set_or_development?
+
+    emails = account.administrators.pluck(:email)
+    subject = 'Your Slack integration has expired'
+    @action_url = "#{ENV['FRONTEND_URL']}/app/accounts/#{account.id}/settings/integrations/slack"
+    send_mail_with_liquid(to: emails, subject: subject) and return
+  end
+end