resolvePage UI helper should return page model instead of virtual file

For consistency, the resolvePage UI helper (introduced in 2.3.0-alpha.2) should return a page model, not the raw virtual file. This will make reusing template partials easier because the current page object and the value returned by resolvePage will share the same properties. It also makes certain information more accessible, such as the version lineage for the page.

For now, the only difference will be that the value returned by resolvePage will not have the navigation property set. This is because the navigation catalog is not currently exposed to the UI. That enhancement will be for a future issue.