Feature/sections add5 cs

add users to section functionality added:
add_to_section function added
javascript for add_to_section
template addition and modifications to fascilitate independent refresh of left and right panes
left and right sides stay in sync between add and delete operations
added enroll_count to the return of section_class_list
unit tests added
added double click protection to add user 

I'm concerned about the assumption here that an sis_section_id of 'None' is the indicator of whether or not a section is editable (i.e. it's a primary section so it's name cannot be edited and the section can never be deleted).  Even if that assumption holds, I think it would be a lot better to pass a context variable to the template that indicates whether or not the section is editable - this variable could, for now, be based on the condition of sis_seciton_id, but if it ever changes then we'd only have to modify one spot instead of changing around multiple templates.