Convert layout.less to CSS

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

Seems rather difficult. There's a few place with escaped CSS syntax, a few places with pure LESS, and a couple mixin uses at the start.

`@line-height-computed` will probably need a discussion to know whether we want it in the XWiki variable set (as API) since from what I can remember it was a workaround implemented for the bootstrap layout but not needed with current CSS.