Add CSS-Diner as an exercise to the web curriculum

[lizTheDeveloper]

Looking to prototype some open-source exercises in the web curriculum.

Add this as an exercise to a relevant workout: https://flukeout.github.io/

Part of the challenge of adding this to the curriculum is figuring out how we'll verify that they did it. Maybe split it up into multiple exercises, send the user the link and tell them to do exercises X-Y, and then ask them the answer to the last one.