Dynamic details form

This pull request cleans up permit_steps#enter_details form view and adds some simple JavaScript to simplify the form.

* Fixes radio buttons to show Yes/No inline
* Organizes content into sections to create better flow through the form
* Adds JavaScript to show/hide contractor details fields
* Rewrites some input labels to make them easier to understand
* Adds some helper text to give context for form fields
* Marks some fields as required