EE: Allow EE to opt out of member notifications

What does this MR do?

This is the EE version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23618

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8424
• https://gitlab.com/gitlab-org/gitlab-ee/issues/8244

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Security reports checked/validated by reviewer