Add Group's GL.com plan name to payload of system hook

In order to apply some validations when we process the hook on the
subscription portal we need to know what's the current plan of Group
that has generated the event.

app/services/system_hooks_service.rb

@@ -130,6 +130,7 @@ class SystemHooksService
     {
       group_name: model.group.name,
       group_path: model.group.path,
+      group_plan: model.group.plan&.name,
       group_id: model.group.id,
       user_username: model.user.username,
       user_name: model.user.name,

spec/services/system_hooks_service_spec.rb

@@ -17,6 +17,8 @@ describe SystemHooksService do
     it { expect(event_data(project, :destroy)).to include(:event_name, :name, :created_at, :updated_at, :path, :project_id, :owner_name, :owner_email, :project_visibility) }
     it { expect(event_data(project_member, :create)).to include(:event_name, :created_at, :updated_at, :project_name, :project_path, :project_path_with_namespace, :project_id, :user_name, :user_username, :user_email, :user_id, :access_level, :project_visibility) }
     it { expect(event_data(project_member, :destroy)).to include(:event_name, :created_at, :updated_at, :project_name, :project_path, :project_path_with_namespace, :project_id, :user_name, :user_username, :user_email, :user_id, :access_level, :project_visibility) }
+    it { expect(event_data(group_member, :create)).to include(:event_name, :created_at, :updated_at, :group_name, :group_path, :group_plan, :group_id, :user_name, :user_username, :user_email, :user_id, :group_access) }
+    it { expect(event_data(group_member, :destroy)).to include(:event_name, :created_at, :updated_at, :group_name, :group_path, :group_plan, :group_id, :user_name, :user_username, :user_email, :user_id, :group_access) }
     it { expect(event_data(key, :create)).to include(:username, :key, :id) }
     it { expect(event_data(key, :destroy)).to include(:username, :key, :id) }
     it { expect(event_data(deploy_key, :create)).to include(:key, :id) }