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

Fix #604

* 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 #582

Closes #576

Closes #521

Closes #533

To override all child pages

Closes #397
Tests from PR #434

Closes #413
Closes #408

Aliases that have no directory nesting sneakily avoid the code path that ensures the directories exist

Closes #485

Local CSV/TOML/JSON loading Tera function

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

Switch to an `Option<usize>` for the serialized value of `rss_items`.
This lets us just set a blank value in the configuration and thereby
include *all* items.

This is a backwards-compatible change; it does not affect the behavior
of existing configurations.

Fixes #468. Closes #471.

Fix #122