Only two classes remain to declare a media structure:
'media' and 'media-body'. 'media-left/right' are guessed by their
position using the flex layout.

Also, the 'media-list' class has been dropped but <ul/> element
should use the 'list-unstyled' class instead.

Like all other components, spacing between medias are now to be
handled explicitely thanks the spacing utility classes.