This commit removes the option to sort by order and also removes
`page.next` and `page.previous` variables.  Instead, pages can be sorted
by two methods `date` and `weight`.  The Tera `reverse` filter will
reverse either of those sorts, so the old `order` behavior can be
achieved by using the `reverse` filter with `weight`.

In place of the `previous`/`next` variables, this commit adds the
`page.earlier`/`page.later` variables (which are set when the page is
sorted by date) and the `page.heavier`/`page.lighter` variables (which
are set when the page is sorted by weight).  These variables have the
advantage of not having confusing semantics when the `reverse` filter is
used.

Parse _index.md files as sections and render them