Consolidate post_update_hook for members into member model

What does this MR do and why?

Consolidate post_update_hook for members into member model

• consolidation of project and group members into one model to further work towards collapsing them into just members.
• improved testing of the mail/hooks at various levels
• moved as per https://docs.gitlab.com/ee/development/ee_features.html#testing-ee-features-based-on-ce-features:
  • ee/spec/models/group_member_spec.rb -> ee/spec/models/ee/group_member_spec.rb
  • ee/spec/models/member_spec.rb -> ee/spec/models/ee/member_spec.rb
  • ee/spec/models/project_member_spec.rb -> ee/spec/models/ee/project_member_spec.rb
• see #391424 (closed)

MR acceptance checklist

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Related to #391424 (closed)