Fix: improve build time (#1358)

* Remove "passive" toggle

PR #1244 introduced the "passive" toggle, but just-the-docs.js subsequently disabled the only styling that used it, so it became redundant.
This removes it.

* Reduce build time for page-dependent CSS

Fix #1323

- Remove `_includes/head_nav.html`.
- Generate page-independent SCSS in `assets/css/just-the-docs-head-nav.css`.
- Link to `/assets/css/just-the-docs-head-nav.css` in `head.html`.
- Disable the above stylesheet in `assets/js/just-the-docs.js`.
- Generate page-dependent CSS in `_includes/css/activation.scss.liquid` and include in `head.html`.

* No override svg rotate

* Disable both stylesheets safely

* Move the site nav to a new include

- Fix the complete site nav
- Move the site nav to `_includes/site_nav.html`
- Cache the site nav
- Uncache `nav.html`

* Move nav and site_nav to _includes/components

* Replace id prefix

* Update breadcrumbs.html

Replace several filters by a single loop through all the pages,
but breaking as soon as possible.

Profiling indicates that this saves up to 50% of the breadcrumbs build time for the filters.

* Update just-the-docs-head-nav.css

Adjust the number of lines to keep

* Update head.html

Remove superflous type.

* Update activation.scss.liquid

Remove a superfluous closing brace.
Adjust layout.

* Use `scssify` to remove nesting

Preliminary profiling indicates that using `scssify` on the small number of nested CSS rules produced by `activation.scss.liquid` is quick enough.

* Update head.scss

Manual attempt at prettier (pending installation in Atom).

* Avoid generation of nested CSS

Local profiling indicated that using `scssify` on each page takes about 1% of the build time.

- Update `_includes/css/activation.scss.liquid` to generate non-nested CSS.
- Remove use of `scssify` from `_includes/head.html`.

* Ignore false positives from validator

Ignores: `:1.810-1.823: error: CSS: Parse Error.` and `:1.811-1.824: error: CSS: Parse Error.`; had to shift things around since the local config overrides the CI flag.

* Inline `_sass/head.css`

* Update CHANGELOG.md

---------

Co-authored-by: Matthew Wang <matt@matthewwang.me>

_includes/components/sidebar.html

@@ -3,10 +3,9 @@
   Depends on: page(?), site.
   Results in: HTML for the side bar.
   Includes:
-    title.html, nav.html, nav_footer_custom.html
+    title.html, components/site_nav.html, nav_footer_custom.html
   Overwrites: 
-    pages_top_size, collections_size, collection_entry,
-    collection_key, collection_value, collection, nav_footer_custom.
+    nav_footer_custom.
   Should not be cached, because nav_footer_custom.html might depend on page.
 {%- endcomment -%}
 
@@ -17,59 +16,9 @@
       <svg viewBox="0 0 24 24" class="icon" aria-hidden="true"><use xlink:href="#svg-menu"></use></svg>
     </button>
   </div>
-  <nav aria-label="Main" id="site-nav" class="site-nav">
-    {% assign pages_top_size = site.html_pages
-          | where_exp:"item", "item.title != nil"
-          | where_exp:"item", "item.parent == nil"
-          | where_exp:"item", "item.nav_exclude != true"
-          | size %}
-    {% if pages_top_size > 0 %}
-      {% include_cached nav.html pages=site.html_pages %}
-    {% endif %}
-    {%- if site.nav_external_links -%}
-      <ul class="nav-list">
-        {%- for node in site.nav_external_links -%}
-          <li class="nav-list-item external">
-            <a href="{{ node.url | absolute_url }}" class="nav-list-link external">
-              {{ node.title }}
-              {% unless node.hide_icon %}<svg viewBox="0 0 24 24" aria-labelledby="svg-external-link-title"><use xlink:href="#svg-external-link"></use></svg>{% endunless %}
-            </a>
-          </li>
-        {%- endfor -%}
-      </ul>
-    {%- endif -%}
-    {% if site.just_the_docs.collections %}
-      {% assign collections_size = site.just_the_docs.collections | size %}
-      {% for collection_entry in site.just_the_docs.collections %}
-        {% assign collection_key = collection_entry[0] %}
-        {% assign collection_value = collection_entry[1] %}
-        {% assign collection = site[collection_key] %}
-        {% if collection_value.nav_exclude != true %}
-          {% if collections_size > 1 or pages_top_size > 0 %}
-            {% if collection_value.nav_fold == true %}
-              <ul class="nav-list nav-category-list">
-                <li class="nav-list-item{% if page.collection == collection_key %} active{% endif %}">
-                  {%- if collection.size > 0 -%}
-                  <button class="nav-list-expander btn-reset" aria-label="Toggle collection {{ collection_value.name }}" aria-pressed="{% if page.collection == collection_key %}true{% else %}false{% endif %}">
-                    <svg viewBox="0 0 24 24" aria-hidden="true"><use xlink:href="#svg-arrow-right"></use></svg>
-                  </button>
-                  {%- endif -%}
-                  <div class="nav-category">{{ collection_value.name }}</div>
-                  {% include_cached nav.html pages=collection %}
-                </li>
-              </ul>
-            {% else %}
-              <div class="nav-category">{{ collection_value.name }}</div>
-              {% include_cached nav.html pages=collection %}
-            {% endif %}
-          {% else %}
-            {% include_cached nav.html pages=collection %}
-          {% endif %}
-        {% endif %}
-      {% endfor %}
-    {% endif %}
-  </nav>
-
+  
+  {% include_cached components/site_nav.html %}
+  
   {% capture nav_footer_custom %}
     {%- include nav_footer_custom.html -%}
   {% endcapture %}