Replace Less with CSS (#23481)

Ran most of the Less files through the Less compiler and Prettier and
then followed up with a round of manual fixes.

The Less compiler had unfortunately stripped all `//` style comments
that I had to restore (It did preserve `/* */` comments). Other fixes
include duplicate selector removal which were revealed after the
transpilation and which weren't caught by stylelint before but now are.

Fixes: https://github.com/go-gitea/gitea/issues/15565

.stylelintrc.yaml

@@ -5,8 +5,6 @@ ignoreFiles:
   - "**/*.go"
 
 overrides:
-  - files: ["**/*.less"]
-    customSyntax: postcss-less
   - files: ["**/chroma/*", "**/codemirror/*", "**/standalone/*", "**/console/*"]
     rules:
       scale-unlimited/declaration-strict-value: null