Retain n megabytes in icloud for things like photo shuffle wallpaper

[an0therd3v]

use case is allow for download all just like the Move use-case, but configure to retain approximately n megabytes in icloud to keep a set of photos for photo shuffle wallpaper.

It would be implemented by calculating the average size of all the photos in megabytes, then taking a ratio of mb_to_retain/(average_size*len(photos)) which gives a % of photos to retain. After that, you use randint(0,100) to decide which photos to retain vs delete.

I have it wired up without tests, but don't have rights to create a PR branch