Allow page layout to be soft set in site config

[mojavelinux]

If the page layout is soft set in the site config (the value has a trailing @), then the page layout should not override the layout set in the front matter or AsciiDoc header.

[mojavelinux]

As part of this change, the default page layout should be computed in load_header and passed to the AsciiDoc processor instead of being prepended to the top of the content. This gives us more control over how the value is resolved.