Update MVC lesson example, add MDN link to MVC

[sertmer]

This PR addressed issue #710

Summary of changes needed:

• [x] The current lesson uses a class based component to describe the MVC pattern
• [x] This example needs to be updated to be a functional component
• [x] Any language in the lesson that is referring to that class based component may also need to be updated
• [x] The lesson should include a link to the MVC pattern architecture page on MDN

Changes Made:

• [x] All of the above

Questions

• Please review my react code - I was feeling a bit rusty!
• No other specific considerations

[sertmer]

Thanks Heather! I made those changes and I'll merge it now!