Backport the Web IDE editor to Libre

The vision for the Web IDE is more than an editor with tabs and commit staging, but we've decided to make the editor available to everyone. More advanced features (for example live web terminal) will likely be added to GitLab Ultimate and will be evaluated case by case.

Proposal

Backport the Web IDE to GitLab CE

Current EE merge requests

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4789
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4623
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4499
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4951 (optional)
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4969