[MM-56904] Reduce the number of api requests made to fetch user information for GMs on page load (#27149)

* use new endpoint to fetch group members

server/channels/testlib/store.go

@@ -76,6 +76,7 @@ func GetMockStoreForSetupFunctions() *mocks.Store {
 	systemStore.On("GetByName", model.MigrationKeyAddIPFilteringPermissions).Return(&model.System{Name: model.MigrationKeyAddIPFilteringPermissions, Value: "true"}, nil)
 	systemStore.On("GetByName", model.MigrationKeyAddOutgoingOAuthConnectionsPermissions).Return(&model.System{Name: model.MigrationKeyAddOutgoingOAuthConnectionsPermissions, Value: "true"}, nil)
 	systemStore.On("GetByName", model.MigrationKeyAddChannelBookmarksPermissions).Return(&model.System{Name: model.MigrationKeyAddChannelBookmarksPermissions, Value: "true"}, nil)
+	systemStore.On("GetByName", model.MigrationKeyDeleteDmsPreferences).Return(&model.System{Name: model.MigrationKeyDeleteDmsPreferences, Value: "true"}, nil)
 	systemStore.On("GetByName", model.MigrationKeyAddManageJobAncillaryPermissions).Return(&model.System{Name: model.MigrationKeyAddManageJobAncillaryPermissions, Value: "true"}, nil)
 	systemStore.On("GetByName", "CustomGroupAdminRoleCreationMigrationComplete").Return(&model.System{Name: model.MigrationKeyAddPlayboosksManageRolesPermissions, Value: "true"}, nil)
 	systemStore.On("GetByName", "products_boards").Return(&model.System{Name: "products_boards", Value: "true"}, nil)