www: fix for running under mount paths

www: fix for running under mount paths

We try to avoid issues like these by using relative URLs
in hrefs, but we can't avoid the problem with Location:
for redirects and Atom feeds which are likely to be
rehosted elsewhere.

We also reorder some of the code to work around a weird
issue on the psgi-plack mailing list:
<20160516073750.GA11931@dcvr.yhbt.net>
(Somewhere on https://groups.google.com/group/psgi-plack
 but it's probably not bookmarkable)