bake/serve: Fix how taxonomy index pages are setup and rendered. * Properly use the taxonomy's setting name where appropriate. * Delete duplicated (and sometimes incorrect) code in 2 places to setup filtering on an index page and consolidate it on the `PageRenderingContext`. * Add unit tests.

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