Honor DISPLAY_CATEGORIES_ON_MENU setting in pelicanconf.py
Removed the Pelican/svbtle links, in favor of a dedicated colophon page
Automatically sort the tags before rendering
Fix broken links on the 'tags' page

Some 'tags' links are broken, because the tag text itself may contain
unexpected characters (whitespace, periods, etc).  Scrub the link targets to
match to the actual file names on disk.

There's probably a better way to solve this.  Perhaps a dedicated filter
for scrubbing tags; or possibly smarter logic in generate_tags() in the Pelican
engine itself; but this works well enough for now.