Add Template Composer button for resetting preview config

During development of a Hippo project, you often find yourself switching between the Console (setting up configuration) and the Channel Manager (testing your changes). Testing the changes creates a preview configuration, which jeopardizes subsequent (iterative) refinement of the configuration.

A new channel manager button (to be activated through additional repo-based configuration [1]) should be added to delete the preview configuration after testing in the Channel Manager.