Notices: Sass @extend directive should not be used inside a @media directive

[lancewillett]

Currently, libsass (and so node-sass) support a buggy version of @extend inside @media, but the spec is not supposed to allow it. libsass will be adhering to the spec in its next version: sass/libsass#712 sass/libsass@d68cc86

http://sass-compatibility.github.io/#cross_media_extend

There are plans to eventually allow these "cross media @extends", but no timeline: sass/sass#1050

We're doing this in alerts:

wp-calypso/client/notices/style.scss

Line 29 in 53bde8c

@extend %noticon;

, which will break after some upgrade of node-sass.

[lancewillett]

CC @rickybanister in case this is part of moving Notices to gridicons.

[lancewillett]

This was fixed in c5f3d59