Keen Modding Live - Version chains

I put together the level versioning system on the assumption that the time uploaded would be sufficient to identify which versions are based on which, but today I decided it would be better to have that information stored explicitly. That way, there can be a tree of versions rather than just a line, and the correct chain can be preserved even if the original author publishes a new version in between someone else starting and releasing an alternative one. The "upload level" API endpoint now takes a baseversion parameter if an existing level ID is supplied.