Feature/sections tool sm2

This pull request contains changes for delete section" 
- added remove_section method that uses canvas_sdk
-prevent primary sections from being deleted
- added ajax call to UI to handle 
    *hide the deleted row on success
    * display success and failure message
    * display special message when primary section is attempted to deleted(even though UI doesn't have a delete for primary sections)
- unit tests
- UI has a 2 delete icons as placeholder -one for deletable sections and a disabled one when member count >0

I'm just wondering if you tried to add these params to the url method above like {% url 'ap:remove_section' sec_id, sis_id %}? according to the docs you can specify params that way.