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 #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

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

Fix #205

This allow to use base_url in robots.txt, to reference a sitemap for
example.

Closes #443

Closes #403

Closes #282

Closes #272

And some tweaks as I write the docs