Run our own Playground server

It has happened a few times that the official playground server is down or slow. When that happened, I've been redirecting the course to use a playground server running on my local machine. This is not ideal since it's slow (especially if 20 people hit it at the same time), but it gave me a fallback in case of trouble.

Running our own playground server could help with

- Capacity: We would be able to scale up the service as needed for our students.
- Latency: If we could get the playground to cache the compiler output (https://github.com/rust-lang/rust-playground/issues/8), then we ought to get better performance on our code snippets by seeding the cache with them.
- Crates: The playground includes the top 100 crates. We could extend this with other crates that we like for the course.

An additional feature which we could consider with our own playground would be the [Aquascope](https://cognitive-engineering-lab.github.io/aquascope/) visualization tool. (Aquascope can embed static images into the output, but we'll likely want the interactive playground experience as well.)