Tests for all of the navigation fixtures

- Also organized the page mapping in header_menu to line up with the appliance, point out missing pages, and just generally make it readable
- Other modifications were fixed when the caused the fixture tests to fail

With this, we shouldn't need to assert fixture.is_the_current_page in just about every test, because
  1. We'll always use the fixtures for high-level navigation.
  2. All the fixtures are tested.