POST to /listFolders is slow

When loading the CodeWorld home page, the slowest part by far is loading the existing folders. This request takes over a second. It's also sent twice, when it shouldn't need to be.

Should figure out why it takes so long. My guess is that the directory listing for data/codeworld/projects is too long, and it might help to shard it by the first few characters, the same way we do for data/codeworld/user. But that's just a guess.