Allow "all" as a house parameter and always include it in the nav link

This makes it consistent and clear that the house name/parameter is
always present.

The only page we're not explicit on is the main index page. All pages
below that are explicit about what they're displaying in the path.
Allow the date parameter to just be a year
Use helper to display year links
Default to showing divisions from the latest calendar year

Instead of displaying the latest hardcoded parliament session.
Only show year nav link as active if it's set

This means "rdisplay" is still supported and sensibly doesn't
highlight a year on the nav bar. Clicking a year busts you out of that
and into the nicer calendar year navigation.