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.
@christianv , we don't want to log a message. A non-JSON string in the payload is just as uninteresting to us as an event without a payload or a JSON string that doesn't include a subject property we handle.

How about if we trim the try/catch block to clarify? E.g.,

````
var message;
try {
  message = JSON.parse(e.data);
} catch (err) {
  // The message is not for us.
  return;
}
````