Mermaid v8.2.3 added support for styling in subgraphs, but if there are
multiple words in a title, quotes are required.

* https://github.com/knsv/mermaid/pull/845
* https://github.com/knsv/mermaid/issues/391#issuecomment-495602059

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65338