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'd prefer to see you using the render mock here instead of actually rendering the error response since that's testing something outside of your method.  The way to do that would be to have an assert on the render_replacement that focuses solely on the status returned, as in:
`render_replacement.assert_called_with(ANY, ANY, {'message' : ANY}, status=500)`
Then in any related test with the same conditions, you can set status to ANY and verify other individual conditions, like the template name, etc.