Add ability to blacklist maps based on md5sum (or alternative solutions)

[Baxxster]

To avoid people just re-uploading maps added to some kind of ban-filter, store the map's hash in a database/file so that exact same version of the map can't be uploaded again.

It won't stop people from editing it and then reuploading it, but it would require more time spent on their end and could mean it's more hassle to keep uploading maps that aren't suited to be on the RC