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).

Screenshots for smaller screens and larger screens.

![ios simulator screen shot 13 nov 2014 10 40 08](https://cloud.githubusercontent.com/assets/417754/5028066/9528a2a2-6b2e-11e4-8f00-fe75e3dc4c3e.png)

![dob-desktop](https://cloud.githubusercontent.com/assets/417754/5028068/9af870e0-6b2e-11e4-8d80-0094ac824860.png)


I'd recommend `min` and `max` attributes where possible. Browsers which show the steppers, change the value with up/down keys and scroll wheel. This means you can easily get to negative numbers. These attributes will help.