Ensure project zipping works well

[Bubballoo3]

I just tried the 'Zip project' button and it failed with Error creating ZIP file: undefined method 'each' for an instance of String. Not sure how prevalent this error is or what caused it, but we should circle back and make sure the whole project->template->copies process is understandable and free of bugs. This is potentially related to #4340

[Bubballoo3]

I discovered the error happens because when the files attr is not provided at update_attrs, it gets defaulted to a blank string, despite being treated like an iterable (hence the 'each' call above).

The deeper error is the fact that as noted in #4340, there is no manifest to get a list of 'zippable' files from the user. So solving this will involve improving the setting pattern of files but also providing a good way to get this variable a value. It likely shouldn't even be an instance attribute, but was lumped with other attributes in the manifest. I feel like a (cacheable?) instance method that checks a file is a better logical step so that it can be idempotent.