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.
I was a bit confused by the `add()` function here.  Based on the jQuery doc, it looks like what's going on here is that the list items are selected, and the spans with class 'remover' are added to that collection.  I'm guessing the desired target of the click handler is the span?  Either way, it might be a little easier to read with a single selector expression like `$(".list-student .remover")` or, if possible, `$(".remover")`