Ship octotree as part of GitLab

Report https://www.reddit.com/r/programming/comments/471he5/gitlab_85_released/d0ae85g "Especially the file tree view is simply a must-have for navigation."

Asked in https://github.com/buunguyen/octotree/issues/265

potential iteration:

• Add dropdown for view sorting based on date, size, name - idea came from https://gitlab.com/gitlab-org/gitlab-ce/issues/25110#note_19231916

/cc @JobV