Share private sgf folders with user

[shinuito]

There’s probably a number of ways the sgf library could be improved (like being able to move files to/from folders).

One thing that might be useful is the ability to share access to private folders with users. Or the ability of toggling folders to/from private etc.

There’s the obvious convenience but also it possibly reduces duplication, and maybe even duplicated ai reviews in some instances, if you had a game in a folder and then had to reupload it and rerun a review to share the review and game with someone.