Allow negative currency

I know that the currency field needs urgent refactoring (e.g. as it does not allow setting the currency symbol, vgl idea: https://community.pipefy.com/ideas/ability-to-use-eur-instead-of-usd-in-any-language-version-of-pipefy-107). When you are on it, I just want you to make sure to also support negative currency values, e.g. to symbol dept etc., for calculations etc. Thanks!