Revert "Revert "enable cloning of DSL-defined levels""

[Hamms]

Reverts #18115

The original PR failed on test:

  test_should_clone                                              ERROR (0.13s)
NoMethodError:         NoMethodError: undefined method `dsl_text' for #<Blockly:0x0000002e781cb0>
            app/controllers/levels_controller.rb:215:in `clone'
            app/controllers/application_controller.rb:145:in `block in with_locale'
            app/controllers/application_controller.rb:144:in `with_locale'
            test/controllers/levels_controller_test.rb:636:in `block (2 levels) in <class:LevelsControllerTest>'
            test/test_helper.rb:117:in `block in assert_creates'
            test/test_helper.rb:116:in `assert_creates'
            test/controllers/levels_controller_test.rb:635:in `block in <class:LevelsControllerTest>'
            test/testing/setup_all_and_teardown_all.rb:22:in `run'