DRY support request new forms

This change:

* makes all 'new request' forms much more uniform. There's a shared 'new' view, and all the request-specific elements are in the 'request_details' partials.
* introduces conditional logic to show guidance at the top of the form, if any is present.
Merge pull request #196 from alphagov/dry-request-views

DRY support request new forms