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.

#!/bin/sh

CUR_DIR="$( cd "$( dirname "$0" )" && pwd )"
CHEF=${CUR_DIR}/../bin/chef
OUT_DIR=${CUR_DIR}/../piecrust/resources/messages
ROOT_DIR=${CUR_DIR}/messages

$CHEF --root=$ROOT_DIR bake -o $OUT_DIR
rm ${OUT_DIR}/index.html