Tidied up the <header> of base.html template

* Removed 'text/javascript' and 'text/css' as these attributes are not required by HTML5
* Re-ordered the stylesheet loading to avoid using !important when over-riding third party CSS
Fixed problems with datepicker not appearing in date fields

* Decided to use jQuery UI datepicker instead of browser native datepickers
  to have greater control over aesthetics of widget
* To support this I've added the datepicker class to various input fields in forms.py
* This changes means I can remove the Modernizr yepnope tests in various templates