Migrate templates to Zola 0.23 / Tera v2

bin/update-zola always installs the latest release, which as of
v0.23.0 rewrote the template engine to Tera v2. That broke the whole
site: removed filters (slice, concat, escape), test calls now require
named args, shortcodes are gone in favor of Tera components, and
content itself is now Tera-templated before markdown runs.

- slice(end=n) -> native [:n] slicing; concat(with=x) -> spread [...arr, x]
- escape -> escape_html; starting_with/containing -> named `pat` arg
- get_taxonomy_url `name` -> `term` (old name is deprecated)
- %+ date format no longer recognized; replaced with explicit RFC3339 pattern
- shortcodes (note/tip/warning/danger/youtube) rewritten as components;
  content call sites updated to component invocation syntax
- {% include %} no longer inherits for-loop-local variables (only ambient
  page/section context) - post-card.html converted to a component that
  takes page/index explicitly instead of relying on implicit `page`/`loop`
- two content code blocks with literal Jinja-style {{ }} syntax (Netlify
  CMS config, a Flask template snippet) wrapped in {% raw %} so Tera
  doesn't try to render them, since content is now templated too
- fixed a real content corruption bug along the way: the youtube
  shortcode's multi-line <iframe> tag had a conditional that could
  collapse to a whitespace-only line, which prematurely closed the
  HTML block under CommonMark rules and got the rest of the tag
  syntax-highlighted as code instead of rendered

Verified with a clean `zola build` and `zola serve` on v0.23.1, plus a
spot-check of the rendered HTML for both callouts and the video embed.
`zola check` only reports pre-existing external link rot, unrelated to
this change.

templates/partials/header.html

@@ -10,7 +10,7 @@
                 {% if config.extra.menu %}
                     {% for item in config.extra.menu %}
                         {% set current = current_path | default(value="") %}
-                        {% set is_active = item.match and current is starting_with(item.match) %}
+                        {% set is_active = item.match and current is starting_with(pat=item.match) %}
                         <a href="{{ get_url(path=item.url) }}" class="nav-item{% if is_active %} is-active{% endif %}"{% if is_active %} aria-current="page"{% endif %}>{{ item.name }}</a>
                     {% endfor %}
                 {% endif %}