Accessibility fixes: make it easier to edit data in form fields

Part of the actions for the feedback in November '14 accessibility review.

https://trello.com/c/LT0lPO5S

* Prevent the cursor jumping around when editing the mortgage term
* Hide the sliders from screen readers on the mortgage calculator
* Convert the mortgage calculator form to use Dough inset labels
* Also convert the stamp duty calculator form to use Dough inset labels
  for consistency
* Move input validation to keypress event for payment term input. We now
  filter keys as they are pressed to prevent non-numeric characters
  being input. This makes the 'years' directive redundant so it has been
  removed
Merge pull request #217 from moneyadviceservice/accessbility_fixes

Accessibility fixes: make it easier to edit data in form fields