nsIWebHandlerApp is a web-based handler, as speced by the WhatWG HTML5
draft. Currently, only GET-based handlers are supported. At some point,
we probably want to work with WhatWG to spec out and implement POST-based
handlers as well.
Template used to construct the URI to GET. Template is expected to have
a %s in it, and the escaped URI to be handled is inserted in place of
that %s, as per the HTML5 spec.