www: add expires attribute to xhr
www: add datepicker to prefix form
www: add logic for expires field
www: fix CSS for datepicker

The .listing class was inadvertently applied to more tables than
intended.

.listing th, td {
    ...
}

Means th element with class listing and all td elements, not just td
elements with class listing.
Merge pull request #658 from plajjan/599-web-expires

Add expires field to web UI