Earn: add option for more privacy

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Currently Jam Earn uses only the feature of "yield-generator-basic.py" from Join Market. It would be a bonus to have the option that uses the feature available in "yg-privacyenhanced.py" of Join Market where the upper and lower amounts and the percentage are randomly created from the variance variables "cjfee_factor" and "size_factor". This feature would make Jam Earn more private.