Create mixin 'opacity' for compatibility reasons

Created by: mep253

Migrating from Bootstrap 3 fails on missing mixin 'opacity'. Instead of the missing one: // Opacity 236: @mixin opacity($opacity) { 237: opacity: $opacity; 238 // IE8 filter 239: $opacity-ie: ($opacity * 100); 240: filter: unquote("alpha(opacity=#{$opacity-ie})"); 241 }

you could add something simple like this, only for compatibility reasons: @mixin opacity($opacity) { opacity: $opacity; }