Tag and category links missing trailing slash

[alanorth]

@Xekon noticed in #127 that tag and category links are missing trailing slashes, which cause issues on some webserver configurations, and redirects on others. The issue is in layouts/partials/meta-terms.html where we iterate over all the tags in the current post's front matter and manually construct a URL out of the resulting string. The lack of a trailing slash is not a bug in Hugo's taxonomy tooling (as here we are avoiding it entirely), but rather a side effect of our current custom implementation.

There are more idiomatic Hugo ways to do this now, for example with .Data.Terms.Alphabetical, although that means we need to manage the "count" a different way so we know whether to put a comma or not when there are multiple terms.

For now I think it's better if we just hard-code a trailing slash.