adding back default in rebuild

Fix #620

* 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 #533

To override all child pages

Closes #397
Tests from PR #434

Closes #413
Closes #408

Close #501

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

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

Closes #443

Fix #412

This commit removes the option to sort by order and also removes
`page.next` and `page.previous` variables.  Instead, pages can be sorted
by two methods `date` and `weight`.  The Tera `reverse` filter will
reverse either of those sorts, so the old `order` behavior can be
achieved by using the `reverse` filter with `weight`.

In place of the `previous`/`next` variables, this commit adds the
`page.earlier`/`page.later` variables (which are set when the page is
sorted by date) and the `page.heavier`/`page.lighter` variables (which
are set when the page is sorted by weight).  These variables have the
advantage of not having confusing semantics when the `reverse` filter is
used.

Closes #275

Closes #272

Closes #235

Tags/Categories only work on dates
and tests were assigning them randomly

Add shortcode for Streamable video

Fix #75

Fix #66