Improve splash nav

- Makes the homepage splash near-full-screen size. The goal is to help users understand that they should scroll down for content. 
- Also adjusts sub-page narrow splash to match.
- Changes background image for a larger, higher-quality one.
- Moves nav to top of screen where uses expect it. Adds home to nav on all pages rather than having it appear only on subpages. The goal of that change is to help users feel more grounded in site nav because it doesn't change.