WIP: Add Group-wide secure variables - FE (Engineering UI/Replicates from Project-wide secure variables)

What does this MR do?

This MR adds the functionality of Group-wide secure variables from FE perspective. The specification of this feature is described at https://gitlab.com/gitlab-org/gitlab-ce/issues/12729#note_28734045. This MR replicates the existed UI functionality from Project-wide secure variables. Therefore, it's only focusing on an engineering UI, but fancy stuff regarding CSS/Javascipt.

Are there points in the code the reviewer needs to double check?

• Is every UI components efficiently organized?
• Is authentication process properly handled?
• Is document updated?

Why was this MR needed?

This MR got at least 50+ votes from users.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11078
• https://gitlab.com/gitlab-org/gitlab-ce/issues/12729

/cc @ayufan

Closes #12729 (closed)