Allow email notifications to be disabled for all users of a group or project

This MR is being continued in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30755

What does this MR do?

Allows administrators of groups/projects to disable email notifications on a per-group or per-project basis, regardless of the users settings

Note that this is only labeled WIP because it lacks tests. It works and is functional in my local GDK environment. Additionally, have been using this in two 10.x production sites since the beginning of July.

Are there points in the code the reviewer needs to double check?

I don't really know ruby (though, I did test this in a GDK environment). I looked briefly at writing tests, and it's a bit confusing. I'd like to make sure that this MR will actually be accepted before taking the time to write tests.

Why was this MR needed?

Some organizations may not want any code or data going out over email.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/50020