Closes (PR) #726 : Error checking to the add section modal

In reference to PR #726

I took what @Ayaya made and did some changes.

 - add padding-bottom to the modal
 - add error checking in the modal
 - add remote requests to do not refresh the webpage
 - change location of select from _student_form to a partial
 - add rjs files to interact with the modal and the select sections