* 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
