Expose has_subgroups for groups endpoint

This is needed because we have to know before hand if the group has subgroups. If the group doesn’t have subgroups we will not let the user to click a group row to fetch subgroups, also we will hide caret icon which is used to indicate the row can be expanded.

app/serializers/group_entity.rb

@@ -26,4 +26,8 @@ class GroupEntity < Grape::Entity
   expose :can_edit do |group|
     can?(request.current_user, :admin_group, group)
   end
+
+  expose :has_subgroups do |group|
+    group.children.any?
+  end
 end