feat: Add sorting component

This MR adds the gl-sorting component as described in gitlab-org/gitlab-services/design.gitlab.com#301 (closed).

The GlSorting component is currently built on top of the GlDropdown and GlButton components, this means that the styling is currently a little out from the expected guidelines. The styling should be updated when the respective styling is updated in the GlDropdown (gitlab-org/gitlab-services/design.gitlab.com#274 (closed)) and GlButton (gitlab-org/gitlab-services/design.gitlab.com#281 (closed)) components.

In addition, the icons used in this component are temporarily using FontAwesome. This is will be changed once the icon component (#98 (closed)) has been completed.

Preview