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

web_src/css/markup/asciicast.css

@@ -0,0 +1,8 @@
+.asciinema-player-container {
+  width: 100%;
+  height: auto;
+}
+
+.asciinema-terminal {
+  overflow: hidden !important;
+}