Merge branch 'support-notifications-on-project-snippets' into 'master'

Support e-mail notifications for comments on project snippets

While working with project snippets recently, I noticed that notifications would not be sent out for comments on notes. This MR fixes this.

Note: I'm not completely sure why `ProjectSnippets#participants` returns an empty array if you don't include the concern that is already in `Snippets` but didn't dig into it any more.

Closes #2334

See merge request !3987