Add CString::StartsWith and CString::EndsWith
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 prepend the prefix on non-absolute redirects