Allow EE to opt out of member notifications

What does this MR do?

EE wants to disable certain notifications in the GroupMember and ProjectMember models. To do so, it defines the "skip_notification" accessor on the Member model and uses this variable in a variety of places. This however will result in merge conflicts any time CE changes these lines.

To resolve this, we add Member#skip_notification? and use this in the places where EE expects it to be. By default this method just returns false, so the CE behaviour remains the same while allowing EE to easily redefine it.

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8730

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