Feature/sections cs

merged changes from develop branch

added remove_from_section function - using canvas_sdk
added ajax call to section_details template to call remove_from_section
added ids as needed to appropriate template elements
javascript to handle decrement of user count, display of 'no users' message, display of success and error messages
new tests
Note that UI is not complete for delete users functionality, so x's for delete and placement/wording of error and success messages are, essentially, placeholders.
Based on the utils method being used in the view, it looked like the enrollments object would never be None, but making it a truth value test here will cover more cases, so I like this change.