Split sitemap when it is getting too big

Closes #607

trim_{start, end} is introduced in rust 1.30.0 and
trim_{left, right} is deprecated since 1.33.0.

* Change the behavior of the template rendering:
    * Check if the template bare name is present
    * Check if the template is part of a theme
    * Fallback to defaults
* Change the behavior of the shortcode rendering:
    * Call the template rendering function
* Prepend `__zola_builtins/` to most of the default elements in `ZOLA_TERA`
* Add a test to verify the presence and content of a `404.html` page
from a theme's template

Closes #576

Fix #546

To override all child pages

Closes #397
Tests from PR #434

Local CSV/TOML/JSON loading Tera function

Close #446
Close #260
Close #478
Close #284
Close #480

Fix #205

Closes #388

Closes #417

Since we are using a builtin template, we can be assured to have the
target file in place. Middleware renamed since it is bound directly to
the concept of handling 404s.

Fix #197