trailing slash page canonicalization

[Jason A. Donenfeld]

Hi folks,

When Christian implemented /about/image.jpg handling, it brought up
the issue of /about vs /about/ and I wound up adding this commit [1]
to canonicalize that url so that rendered content would always wind up
referencing the correct relative path.

We have another potential issue, however. git.zx2c4.com/cgit and
git.zx2c4.com/cgit/ have the same content, yet different URLs. This
might be looked upon unfavorably by search engines. One rectification
is to add a meta tag. Another, and one I'd like to consider here, is
to always add the trailing slash to pages with the same 301 trick as
[1].

I'm interested in hearing some thought and considerations (and even
diffs if someone beats me to it?) before I merge this though.

Regards,
Jason

[1] https://git.zx2c4.com/cgit/commit/?id=d7034806a4b1279f62d606501f831dcad31798e6