Add URIPrefix listener option
Add prefix on redirects and strip it on requests
Prepend URIPrefix to web interface links

Prefix links in templates with URIPrefix and add the input field to the
Listener editor.

The URIPrefix is provided as a top-level template variable. All URIs
have been changed to have the prefix prepended.
Use absolute URIs in Redirect

Although most browsers seem to accept relative redirects, it is not
allowed by HTTP/1.1.

This commit changes relative URIs to absolute ones when redirecting.
Don't support relative redirects
Redirect to URIPrefix for paths outside the prefix
Set favicon URL to respect the prefix

When not specified, browsers tend to send favicon request to
/favicon.ico. This commit tells the browsers to look for it under the
URIPrefix.