Use global isSafeURL utility in safe_link component

This MR follows up on these MRs:

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9767
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28943

What does this MR do?

The isSafeURL utility has been moved to global URL utilities, in app/assets/javascripts/lib/utils/url_utility.js. This prior refactor allows us to remove the previous ee-specific utility, as well as its specs file, all specs having been moved into spec/frontend/lib/utils/url_utility_spec.js.