Add trailing slashes to internal URLs

The move to GitHub Pages/Jekyll has meant trailing slashes on all urls
(even those that ended with .html!), so update internal links to cut out
extra HTTP redirects.
Fix up tag URLs

Tag URLs have changed since moving to GitHub Pages/Jekyll; they're now
all lowercase with dashes instead of "funny" characters.
Put post excerpts back in

Previous version was a hack (whole post, truncated with CSS), but Jekyll
has nice support for these :-)