Revert "De-remify mixins and extends"

This reverts commit 9f25cb3165fe7560b64e778be98fd9d4a9033469.

Conflicts:
	common/app/assets/stylesheets/module/_discussion-mixins.scss
Revert "Remove call to rem mixin in icon sprite CSS"

This reverts commit 63a11714b3e14ff253dc5880c7f25670c57291e1.