CLC-4374 Use Canvas's tool_inline support to resize LTI iframe

https://jira.ets.berkeley.edu/jira/browse/CLC-4374

Instructure now provides built-in support for eliminating nested scrollbars in LTI tools, and complains in the JavaScript console if customers continue to use their own event listeners.

Also added comments for the remaining bits of our event handler.
@raydavis that would be a good solution and will it make clear when we're debugging.