Yield donations corrections and improvements

- Always allow canceling when effective donation is <= 0 ADA
- Move the donated amount in NFT datum from extra plutus data into
  metadata map (as "donated")
- Correct the recorded donated amount to use donation amount in ADA and
  not sOADA
- Add timestamp to NFT datum to mark the day the donation was committed
- Add and update tests for the above

Author: danielfarrelly