Use empty state region component for design management empty state
Problem to solve
The empty state added in design management (see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10599) could not use the empty state region component because the component could not support custom actions (see gitlab-ui#223 (closed)). See also discussion in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10599#note_157867305.
Further details
Custom actions are now supported (see gitlab-ui!242 (merged)).
Proposal
Replace the empty state code in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10599/diffs#8af096d5d214672087329886074c9ee082ad0487_0_27 with the empty state region component.