Media queries should be in em not rem

Since all media queries level one units are relative to the root element, setting the media queries in rems instead of ems, is useless and reduces the support for media queries in browsers like safari 5 (which do support rems but not in media queries)

Start by locating the media-query conversion logic in the PostCSS plugin and reviewing any tests covering media queries. Update the behavior so generated media queries use em units rather than rem units, then verify the existing test suite covers the browser-compatibility case.

Written by the indexing model from the issue text.