Expiration date calculation aid using percentages

[lentschi]

Originally brought up by @sap7772000zilla - see composite issue #1 in the wrapper repository:

T-X%: is not always possible to consume in one day canned beans with whip cream with salami and fresh pineapple...
Suggestion4: please consider generating a dummy expiry date earlier than the original to provide the user some buffer time for consumption: the user should select the X% and the app should automatically change the expiry date using the formula T(original date)- X%.
For example if expiry date is 2 months and X is 10 then the calculated expiry will be
60 - 6(=10% of 60) = 54 days -> this will be adopted by the system.

Should be made optional by #13