Handle project images when remixing #48
Description
When a project is remixed it should include any images from the original project.
We either duplicate the images for the new project, or point the new project at the same images as the original.
If we point at the same resources we will need to ensure images can't be deleted while still referenced by other projects.
We will then need a regular background job to clean up orphan images.
We would also need to ensure the rake task setting up initial projects does not delete images, it can remove the reference to an image if it is removed. It could also check if an image is already attached (by checksum for example) and not process if so.