Add On-demand Scan empty state illustration
This adds the empty state illustration for the upcoming On-demand Scan feature.
- Relevant URL in the review app: http://add-ondemand-scan-empty-state-illustration.46.101.145.25.nip.io/gitlab-svgs/illustrations?q=illustrations%2Fempty-state%2Fondemand-scan-empty.svg
- Related sub-issue: gitlab#218680 (closed)
- Related parent issue: gitlab#218465 (comment 348373945)
Closes #124 (closed)
Edited by Paul Gascou-Vaillancourt