Show visual feedback when tours become outdated

[gracegtaylor]

Some developers have requested to receive a visual notification when CodeTour becomes outdated, so that they can change the tour or dismiss the prompt.

We can look into implementing a non-intrusive way to notify the user of a change in CodeTour when writing code.

[lostintangent]

Closing this in favor of #141, since we're going to auto-update the steps when the associated code changes.