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!
同意jihao的。这个不是best practice, controller 里面逻辑尽量简单，需要的参数以param方式传递，杜绝使用class variable。

还有如果可以使用activeRecord的interface method, 尽量使用。我没有读model和database scheme，不确定这里是不是必须使用find_by_sql。