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

package.json

@@ -28,8 +28,6 @@
     "jquery": "3.6.3",
     "jquery.are-you-sure": "1.9.0",
     "katex": "0.16.4",
-    "less": "4.1.3",
-    "less-loader": "11.1.0",
     "license-checker-webpack-plugin": "0.2.1",
     "mermaid": "10.0.2",
     "mini-css-extract-plugin": "2.7.2",
@@ -64,7 +62,6 @@
     "eslint-plugin-vue": "9.9.0",
     "jsdom": "21.0.0",
     "markdownlint-cli": "0.33.0",
-    "postcss-less": "6.0.0",
     "stylelint": "15.2.0",
     "stylelint-declaration-strict-value": "1.9.2",
     "svgo": "3.0.2",