fix(giscus): theme and language (#783)

* fix(giscuss): system theme

* fix(giscuss): language

* fix(giscuss): theme switching

* chore: remove confusing comment

* fix(giscuss): use a uniq ID

* chore: remove useless function

* chore: use extra language code

* chore: add comment about hash

* Update layouts/_partials/components/giscus.html

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Co-authored-by: Xin <5097752+imfing@users.noreply.github.com>

exampleSite/hugo.yaml

@@ -31,7 +31,7 @@ languages:
     title: هگزترا
   ja:
     languageName: 日本語
-    languageCode: ja
+    languageCode: ja-JP
     weight: 3
     title: Hextra
   zh-cn: