MediaOffline ID and file name does not match naming convention

[jawinn]

When pulling in and using the latest 4.0.0 release into our development Spectrum 2 CSS branch, I noticed that the Media Offline icon was not rendering from the sprite sheet because of the way its ID is named. It also looks like the SVG file itself is not correctly named.

1. It looks like the S2_Icon_MediaOffline_N.svg file should be S2_Icon_MediaOffline_20_N.svg to match the naming of the other icons.
2. Sprite sheet: <symbol id="icon-media-offline-n" should also be <symbol id="icon-media-offline", to match the naming convention of the rest of the icons. That way we can continue to parse the icon names and match them to their IDs.