Move skip_notification from EE to CE in Member/ProjectMember/GroupMember

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EE currently has an additional skip_notification accessor in https://gitlab.com/gitlab-org/gitlab-ee/blob/dc62ac84d0e0b7705e3cdb0346c305eeb11233ec/app/models/member.rb#L10 and subclasses use it in various notification methods such as https://gitlab.com/gitlab-org/gitlab-ee/blob/dc62ac84d0e0b7705e3cdb0346c305eeb11233ec/app/models/members/project_member.rb#L104.

I think skip_notification is generic enough that we could just port it to CE and reduce the differences between CE and EE.