Improve date pattern

(Following discussion on the
Hackpad)[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
Add min and max attributes to date input fields
Create a date snippet

This appears in more than one place, create a snippet to keep both
examples up-to-date.
Also create a date pattern page.
Amend example to show UK date format

- Remove leading zeroes from example date
- Use example for a date which corresponds with min-max attributes and
uses UK date format
Merge pull request #47 from alphagov/remove-native-date-picker

Improve date pattern