Convert general.less to CSS

For a performance improvement, we should convert general.less to a CSS.

As far as I can see, this is the only time where we use https://lesscss.org/features/#extend-feature in XS code (excluding our bootstrap fork ofc). This might be troublesome to convert this in native CSS...

There's a couple mixin uses for link decoration that would be difficult to convert without CSS mixins.

Seems like a moderately difficult conversion.