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 , you're right to flag empty catch blocks. I was trying to learn from Canvas's (IMO) mistake here. We do not own the outer frame of the Instructure Canvas instance, and we do not control all pieces of software which might send a message to the window. Instructure-supplied code or LTI tools from other sources might signal the window as well. And so we can't be sure that every message received by this listener will be packaged as a JSON string. If the payload is in some other form, that doesn't mean it's broken -- it just means that someone other than us packaged it.

Does that make sense? If so, I'll add a comment to the code.