Allow for CSS variables to be used in media queries

Currently, media queries cannot be used in media queries.
This is forcing us to use less whenever a part of a media query needs to be a customizable variable. For instance:

@media screen and (max-width: @screen-xs-max) {}

Resources:

• explanations: https://stackoverflow.com/questions/40722882/css-native-variables-not-working-in-media-queries
• not implemented proposal: https://css-tricks.com/can-we-have-custom-media-queries-please/