Make App._download_files() a static method

[johnclary]

App._download_files() is in fact a static method. We should decorate it as such and drop self in case someone wants to use it as such. Say, for example, you're doing some crazy file migration and you want to monkey patch knackpy for assistance. Me IRL.

I guess it might be more elegant to move the method into Knackpy.api, since that's where the rest of our low-level API methods exist.

The current workaround is to just construct an app instance and call the method manually, which isn't hard.

knackpy/knackpy/app.py

Line 425 in 3ab9c29

def _download_files(self, downloads: list):