Move sound files elsewhere when deleting a sound

[ffont]

Currently when we delete a sound individually we create a DeletedSound object but keep all related files (audio files, previews, etc). We should give a new treatment to those files to that:

• Files are stored in a different location and are not statically served by nginx (this basically applies to previews and displays). Then we should add a locations method to DeletedSound so that we can easily get the paths for the files corresponding to deleted sounds.
• When the reason for deleting a sound is that it is illegal, files should be completely deleted from our servers.
• Make sure files in the new location are correctly mirrored if mirroring option is on.

This should be done after #1383 as otherwise we don't have deletion reason information.