Add Board Approval Date to Valuation Obj

[JSv4]

Per Issue #56, importing comments / suggestions to our static Google Doc into GitHub for discussion

@ashawber suggests including a field for board_approval_date in the Valuation objects as these are required for US 409A valuations.

Since I know we wanted to support more than just the US and 409A in particular, I could see not adding fields specifically for a single valuation type and jurisdiction. That said, given that US DE C Corps are our initial target and almost all of them will have 409A valuations that need to track board approval, I'd propose making this an optional field.

[JSv4]

Converted to issue. Adding field.