feat(badge): Update styling of text badges

BREAKING CHANGE: This implements the [text-only][1] versions of the
latest badge designs.  Support for icons will be added in a future
version.

 - This removes the `primary`, `secondary`, `light` and `dark` variants
   of GlBadge, and adds the `muted` and `neutral` variants.
 - The `pill` prop is now ignored, and all GlBadges are of the `pill`
   type.

It's recommended to replace usage of the removed variants with the new
variants:

    primary   -> info
    secondary -> neutral
    light     -> muted
    dark      -> neutral

[1]: gitlab-org/gitlab-ui#481