wire up shorter, less ambiguous URLs

wire up shorter, less ambiguous URLs

We will prefer URLs without suffixes for now to avoid ambiguity
in case a Message-ID ends with ".html", ".txt", ".mbox.gz" or
any other suffix we may use.

Static file compatibility is preserved by using a trailing slash
as most servers can/will fall back to an index.html file in this
case.

For raw text files, we will follow gmane's lead with "/raw"