Bring back /gitlab-com/settings

For details see https://gitlab.com/gitlab-com/infrastructure/issues/3835

MRs to be merged in this exact order:

• Migrate GitLab.com settings page to the main docs https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17968
• Expose GitLab.com settings page to the docs site https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/217
• Change link under /pricing !10674 (merged)
• Create new redirects in Nginx https://gitlab.com/gitlab-cookbooks/cookbook-about-gitlab-com/merge_requests/40