UI for Dependency proxy for containers

What does this MR do?

Adds UI for dependency proxy feature. Allows enable/disable dependency proxy for containers on group level. Also shows storage taken by cached images.

CE backport MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27965/diffs

---

⚠ Merge only after https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9750 ⚠

Screenshot

What are the relevant issue numbers?

UI for https://gitlab.com/gitlab-org/gitlab-ee/issues/7934

Links

• Issue - https://gitlab.com/gitlab-org/gitlab-ee/issues/7934
• Backend - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10676, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10681, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9750
• UI - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10386
• Docs - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12210
• Config - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10675, omnibus-gitlab!3206 (merged)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer