Support Verisign's OpenID Seatbelt extension

[James Henstridge]

Verisign produced a Firefox extension known as "Seatbelt" to reduce opportunities for identity theft. Details of the extension can be found here:

    https://pip.verisignlabs.com/seatbelt.do

The extension appears to be configured by a <link> element on the base page of the domain. For example, on https://pip.verisignlabs.com/:

      <link rel="seatbelt.config" type="application/xml" href="https://pip.verisignlabs.com/web/brand/default/seatbelt/seatbeltcfg.xml" />

The configuration file includes the following:
 * descriptive information about the OpenID Provider
 * a link to the login page (so we'd need to do bug 176845 to support this)
 * a link to an XML page that indicates the user's login state.
 * details of the OP's SSL certificate (domain, common name, SHA1 hash of certificate).
 * branding for the login status indicator (colours and images).

Once bug 176845 is implemented, it should be fairly trivial to implement this.