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/svg.css

@@ -0,0 +1,9 @@
+.svg {
+  display: inline-block;
+  vertical-align: text-top;
+  fill: currentcolor;
+}
+
+.middle .svg {
+  vertical-align: middle;
+}