TST: Update answer key checksums on upload.

Automate the updating of the answer key when a new key is uploaded
to S3, so that keeping the latest value in sync is more likely.
TST: Improve cumulative risk metrics sheet in answer key.

Update answer key for cumulative risk:
- Annualization of Sharpe
- Use 10 year period
- Use of daily returns vectors instead of compounded return scalar.

No tests or risk module code are currently reading off of this sheet,
but developing it ahead of work in risk module so that the sheet can
be examined and vetted.