internal: Improve handling of taxonomy term slugification. This paves the way to bring slugification options like transliteration to PieCrust. This change mostly makes sure we use one-way slugification, which means, for serving/previewing, we need to slugify each page's terms to compare to the stuff captured from the URL. It also simplifies things a bit in the code.

[pytest]
norecursedirs = .* _darcs CVS .svn .git .hg *.egg-info dist build node_modules venv tmp* {args}