hgweb: compatibility of hgweb.cgi template with Python 3

When running Mercurial on Python 3 hgweb expects the config variable to be a
bytestring.
The solution proposed by Anton Shestakov is to use a byteprefix to the literal
string defined cgi script as provided in this patch for hgweb.cgi,
contrib/hgweb.fcgi and contrib/hgweb.wsgi.
I am not sure whether this covers all relevant templates, but I couldn't find
others.

Differential Revision: https://phab.mercurial-scm.org/D8978