LPS-42537 Apply RTL to non-theme CSS

Hey Brian,

You were right about the sorting of main_rtl.css. It should follow the sorting pattern for SASS, and not for plain CSS. I've sent a PR to Nate. I've also noticed that we need to apply RTL when serving CSS resources that do not belong to themes (for example, those in AUI widgets). This is the required change.

Thanks!