Main material feature

Hi,Dr Ji

     I have made some progress in dynamic-select-menu.

     The three level dynamic-select-menu has been completed on frontstage.

     I also did some work in app/admin/main_material.rb

     Next step,i prepare to improve the dynamic-select-menu by using json data and make it work on backstage

     Thanks!
不建议，你还是用json吧，否则你前台的逻辑一塌糊涂