chore: Introduce enterprise edition license (#3209)

- Initialize an "enterprise" folder that is copyrighted.
- You can remove this folder and the system will continue functioning normally, in case you want a purely MIT licensed product.
- Enable limit on the number of user accounts in enterprise code.
- Use enterprise edition injector methods (inspired from Gitlab).
- SaaS software would run enterprise edition software always.

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

lib/custom_exceptions/account.rb

@@ -28,4 +28,10 @@ module CustomExceptions::Account
       I18n.t 'errors.signup.failed'
     end
   end
+
+  class PlanUpgradeRequired < CustomExceptions::Base
+    def message
+      I18n.t 'errors.plan_upgrade_required.failed'
+    end
+  end
 end