Tie tax credits and rebates to the year income is assessed on

Per conversation in #5356, we currently assign some credits and rebates to the year when they are paid out. For consistency with standard credits (EITC is always paid the year after the income determination) and to ensure we properly attribute marginal tax rates (even if retrospective), we should change this to use the year the income is based on.

Some Covid rebates could be based on multiple years of income. In this case we should model that properly by looking up the prior year's income.