Fix how binary files are handled during external grading

[nwalters512]

For text files, eagerly decoding the base64 contents to a string was fine. For binary files (tar archives, specifically) that breaks things. We now decode the files into a Buffer and write that directly to disk.

[trombonekenny]

Is this a good place to hook in a test case for external grading that the supplied file actually matches what's seen on the grader?

[nwalters512]

We should definitely have a test case for that, though I'm still not sure what the best way to do that kind of integration test for external grading would be. Testing async things is tricky.

[nwalters512]

To be fair, we could probably just test buildDirectory directly if we can get the right data in place beforehand.

[nwalters512]

Scratch that, the function doesn't even read from the database! I'll pretend like I thought ahead to make it easily unit-testable. Let me see if I can write some tests for this.

[trombonekenny]

lgtm

[nwalters512]

I'll punt the tests to another branch, don't know how long those will take me and I want to ship this soon for STAT 430.