resolves #47 fix 404 layout
- update relativize to return to if from is falsy
- consolidate the from value resolution in the find function
- scope DOM queries for menu elements to the menu panel
- short-circuit navigation menu logic if navigation menu is absent
- assume a single nav menu in the navigation script
- don't output breadcrumbs ul if page.breadcrumbs is not set
- don't output navigation menu if page.navigation is not set
- don't set data-component and data-version if page.component is not set
- auto-activate explore panel if page.component is not set; don't show current component info
- add title and contents for 404 page
- use shared main partials in 404 layout
- rename current class to context in navigation explore panel