display logic for Prev FY Carry Forward Card

[fpigeonjr]

If a multi-year CAN, this read only amount will appear on funding edit pages for all years after the first year. It is whatever money is left of the budget.

[fpigeonjr]

• A CAN is multi-year if its can.funding_details.active_period != 1
• The first year of a CAN (in this context) is its "Appropriation Year" which is can.funding_details.fiscal_year
• For a given FY y and CAN x, the carry forward value to display in the UI for a CAN should be the available_funding returned from /can-funding-summary with query params fiscal_year=y - 1 and can_ids=x if y > can.funding_details.fiscal_year else there is no carry forward value to display

👆per @johndeange