[#1065] Refactor the way SuperAvatar is retrieved.

Problem description
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We need to remove IOSUsers.getSuperAvatar and Avatar.getCopy() since that API is broken.

Changes description
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getSuperAvatar(from=avatar) was removed together with Avatar.getCopy().
That code was very ugly ... especialy the getCopy part.

This is a old tech dept and it was also blocking the upgrade to Python 2.7

I tried to provide a dedicate SuperAvatar .

The implementation of SuperAvatar on Windows is not correct... but since there is no such thing as "root" account on windows, I don't know how to do it better.


How to try and test the changes
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reviewers @alibotean 

Nothing special here. Just run the tests.

Let me know if you smell anything fishy here :)