align templates with UI model

align templates with UI model

- align templates with Antora's UI model
- add relativize helper to relativize a root relative URL to the current page
- add detag helper to strip HTML tags from content
- detag page title in HTML title element
- replace the term "domain" with "component"
- rename navigation-component to navigation-menu
- update navigation JavaScript to activate path to current page in navigation tree
- update build preview task to assign correct properties on model
- update sample UI model
- remove page-versions and has-versions helpers
- compare selected/latest/current using equality instead of checking boolean property