Split sitemap when it is getting too big

To ensure we will get the right `content`
directory.
Fix #629

adding back default in rebuild

Fix #620

Closes #607

Closes #533

Closes #385

To override all child pages

Closes #397
Tests from PR #434

Closes #413
Closes #408

Fix #122

Fix #205

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.

Closes #241

Closes #235