Revert "UG LaTeX: typesetting of formulae using standard syntax (google#1858)"

Used `git revert HEAD`:

- This reverts commit 9815271.
- For context, see google#2189

Author: chalin

layouts/partials/scripts.html

@@ -1,5 +1,13 @@
-{{ $needKaTeX  := or .Params.math .Site.Params.katex.enable .Params.chem .Site.Params.chem (.Page.Store.Get "hasKaTeX") (.Page.Store.Get "hasmhchem") -}}
-{{ $needmhchem := or .Params.chem .Site.Params.katex.mhchem.enable (.Page.Store.Get "hasmhchem") -}}
+{{ $needKaTeX  := or .Site.Params.katex.enable .Params.math .Params.chem -}}
+{{ $needmhchem := or .Site.Params.katex.mhchem.enable .Params.chem -}}
+{{ if ge hugo.Version "0.93.0" -}}
+  {{ $needKaTeX = or $needKaTeX (.Page.Store.Get "hasKaTeX") (.Page.Store.Get "hasmhchem") -}}
+  {{ $needmhchem = or $needmhchem (.Page.Store.Get "hasmhchem") -}}
+{{ else -}}
+  {{ if or $needKaTeX $needmhchem -}}
+    {{ warnf "Outdated Hugo version %s, consider upgrading to make full use of all theme features" hugo.Version  }}
+  {{ end -}}
+{{ end -}}
 
 {{ if .Site.Params.markmap.enable -}}
 <style>
@@ -26,8 +34,27 @@
   <script src='{{ "js/deflate.js" | relURL }}'></script>
 {{ end -}}
 
-{{ if $needKaTeX -}}
-  {{ partial "scripts/katex.html" (dict "mhchem" $needmhchem) -}}
+{{ if  $needKaTeX -}}
+{{/* load stylesheet and scripts for KaTeX support */ -}}
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.16.19/dist/katex.min.css"
+      integrity="sha512-6a/jH+OTfV56Vs9Mm+Kl7ZsXWJRM2+EV+JDrW7HLHaf4OU2+9eLkHKFbzpSSbfJ5b60m+7/tQAevrzu7NP7Q2g==" crossorigin="anonymous">
+  {{/* The loading of KaTeX is deferred to speed up page rendering */ -}}
+<script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.19/dist/katex.min.js"
+        integrity="sha512-FkmlbBiAj8fgfoZ8zJ+bZQCU5pdU55irXmJzFF/mXlSvBr2x/gmyLHaRu5iSpcA5Hw6CMLCCFSjB1GmgtNL2Qg=="
+        crossorigin="anonymous">
+</script>
+  {{ if $needmhchem -}}
+  {{/* To add support for displaying chemical equations and physical units, load the mhchem extension: */ -}}
+<script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.19/dist/contrib/mhchem.min.js"
+        integrity="sha512-Cl4bz7Rt9ppgcBOQa4z96q3UqAzHgBzNDm66u2+lDe0ZJQu0Fgz+6jO57E+IZqzlhGzfNBY++pg7Ue+iopaLrg=="
+        crossorigin="anonymous">
+</script>
+  {{ end -}}
+  {{/* To automatically render math in text elements, include the auto-render extension: */ -}}
+<script defer src="https://cdn.jsdelivr.net/npm/katex@0.16.19/dist/contrib/auto-render.min.js"
+       integrity="sha512-6bKDPShiPRjAcdeO8T6cedaVEi0pavPbzue/G/ZRiwVlurWZOSZ/vI9fr2lhk8IPXK7z51AZac+zBvZzgDJzDQ==" crossorigin="anonymous"
+       {{ printf "onload='renderMathInElement(%s, %s);'" (( $.Page.Site.Params.katex.html_dom_element | default "document.body" ) | safeJS ) ( printf "%s" ( $.Page.Site.Params.katex.options | jsonify )) | safeHTMLAttr }}>
+</script>
 {{ end -}}
 
 {{ $jsBs := resources.Get "vendor/bootstrap/dist/js/bootstrap.bundle.js" -}}