Validation for scheme dates

www.agileplannerapp.com/boards/173808/cards/4294

derived from discussions with Paul H:
scheme dates should not be earlier than 1990,
and start date should not be later than end date.

/cc @bishboria @fofr 
As a user that enters "99", seeing "End date can't be earlier than 1900" as a validation message is still confusing.