Implement an improved import process.

Products and sales can be imported from the same interactive page. File's are checked client side for the correct file type, name, age (newer than last import), and uniqueness (an adjustable number of most recent hashes, currently the 5 most recent of each type, are sent with the page when it loads and used for a quick client side uniqueness check). After passing client side validation, a preview of the file is displayed to the user on the page. When the user clicks submit, the form is POST'ed with AJAX, validation is repeated on the server and, upon success, records are imported into the database. Appropriate error and success messages are displayed to the user as needed.