Enable Import/Export for non-admin users

What does this MR do?

Removes the admin restriction and reverts https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1983

This is to be enabled after we are confident enough about the Import/Export security.

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/21315

Related https://gitlab.com/gitlab-org/gitlab-ce/issues/20857 and https://gitlab.com/gitlab-org/gitlab-ce/issues/20821/