Improve date pattern

[Following discussion on the Hackpad about dates](https://designpatterns.hackpad.com/Dates-vpx6XlVjIbE) and a conversation with @matharden about user research from Visit someone in prison, keep three inputs for date. 

Use the pattern and number attributes to trigger a numeric keyboard and hide the “stepper” using
css.

This removes the need for Modernizr, so this script is deleted.

Also reformat the _form_date.scss file to [meet the GOV.UK style guide
recommendations for Sass nesting](
https://github.com/alphagov/styleguides/blob/master/css.md).