internal: Refactor config loading some more. * Remove fixup code in the app to make the app config class more standalone. * Remove support for old-style variants... maybe bring it back later. * Try and fix various bugs introduced by subtle config value overriding order changes.

---
in:
    pages/foo.md: |
        Foo!
    pages/bar.md: |
        Bar!
    pages/allpages.md: |
        {% for p in site.pages -%}
        {{p.url}}
        {% endfor %}
outfiles:
    allpages.html: |
        /allpages.html
        /bar.html
        /foo.html
        /