Allow imports to be nested in a directory

[joshwcomeau]

When uploading a map, Beatmapper expects it to be a direct .zip of the files, NOT in an enclosing folder. I've made the mistake a few times now where I zipped the enclosing folder, and Beatmapper throws a confusing error.

Ideally I should allow imports to be in a directory. At the very least, I should improve the error messaging.