Include file URL in form submission notification

When forms contains a file upload, it's more useful to notify the file URL instead of the file ID.
This PR adds the "answerDisplay" item to the $questionAnswerPairs array. It contains the same value of the "answer" item, except when the field is a file uploaded: in that case it contains the file download URL.

Bugtracker: http://www.concrete5.org/developers/bugs/5-6-2-1/form-block-file-upload-issues/