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.
OK, so would it be fair to say that if we're using the course_instance_id to map to our database then a numeric check makes sense, but if we're using it to communicate back to Canvas through their APIs - as is the case here - then it doesn't matter if it's numeric?