Don't save anything that's too large, or doesn't look at least a little bit like Haskell

I've seen a few situations where people are using CodeWorld to upload and share arbitrary files. Obviously, this isn't the intended use of the platform. To fix this, I should really build a filter that rejects source that just doesn't look anything like Haskell. It's still important to allow code with errors, but if it looks like HTML or binary data, it shouldn't be saved.

At the same time, anything over a reasonable size (say, 500K? but I can look at what's there and find the right threshold) should be rejected.