Add support for deleting the preview configuration to MountResource

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 REST service for deleting a channel's preview configuration is a means to create a Channel Manager button for (locally!) deleting the channel's preview config after testing.