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.

language: python
python:
  - "3.4"
install: "pip install -r requirements.txt"
script: py.test