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/head.html

@@ -4,7 +4,7 @@
     site.search_enabled, site.static_files, site.favicon_ico.
   Results in: HTML for the head element.
   Includes:
-    head_nav.html, head_custom.html.
+    css/activation.scss.liquid, head_custom.html.
   Overwrites: 
     ga_tracking_ids, ga_property, file, favicon.
   Should not be cached, because included files depend on page.
@@ -16,7 +16,11 @@
 
   <link rel="stylesheet" href="{{ '/assets/css/just-the-docs-default.css' | relative_url }}">
   
-  {% include head_nav.html %}
+  <link rel="stylesheet" href="{{ '/assets/css/just-the-docs-head-nav.css' | relative_url }}">
+  
+  <style id="jtd-nav-activation">
+  {% include css/activation.scss.liquid %}
+  </style>
 
   {% if site.ga_tracking != nil %}
     {% assign ga_tracking_ids = site.ga_tracking | split: "," %}