Closes #607

* 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

This fixes #569 .

`markdown_to_html` is heavily refactored, header-related things is
handled in a second pass.

Close #501

Fix #205

Fix #412

Closes #282

Closes #275

Closes #272