Validation for scheme dates

www.agileplannerapp.com/boards/173808/cards/4294

derived from discussions with Paul H:
scheme dates should not be earlier than 1900,
and start date should not be later than end date.

/cc @bishboria @fofr 
@alext - fair point. i've clearly gone astray with this story. corrected in https://github.com/alphagov/govuk_content_models/commit/c704ed8b618ca36dde4d8d68fa08502a87e32deb