Changes to Create Product form in dashboard

[jgmorse]

• In the help text for Identifier field, change the word "handle" to "ID", since it doesn't have to be a handle.
• For the Purchase field, change the label to "Purchase URL".
• Add an optional Display Name field that maps to a text field in the database record, to contain a human-readable title for the product. (This should also display in the list of Products in the dashboard.)
• Add an optional Description field that maps to a text field in the database record, to contain a human-readable description of the product. (This should not display in the list of Products in the dashboard.)