zipfile submissions with all the files in a single subdirectory don't extract properly

[mjmusante]

The comp code expects the zipfile to contain the hierarchy of files that are required to play online. Sometimes authors will zip this up as a subdirectory within the zip. The Play Online feature doesn't handle that.

For example, we expect:

mygame.zip:
index.html
assets/
assets/welcome.png
assets/music.ogg

But some authors will submit a zip that looks like:

mygame.zip:
MyGameName/index.html
MyGameName/assets/
MyGameName/assets/welcome.png
MyGameName/assets/music.ogg

We should be able to detect, when we extract the files, that there's a single directory inside of content and move everything up a level