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 

This is different logic than appears elsewhere in the codebase, like when the full section list is shown or when deciding whether or not to show the addUser links in the template.  Are we sure this logic captures all of the cases, including x-reg courses, etc.?