Integrate exercise submissions with CodeSandbox/CodePen/JSBin

[iampava]

As our editor doesn't run code, students need to implement the exercise separately (maybe on their local machine, maybe in an online editor like CodeSandbox, CodePen etc).

Let's integrate our editor with one (or all) of these so that the user only uploads the link to the solution. Behind the scenes we use APIs or we crawl the website to get the solution for teacher feedback.