Domino effect on video deletion

[susanodd]

This is related to #1453

Because there still exist NME and Perspective videos that point to the "normal" video, the delete video for these subclasses ought to be revised.

The Python command os.unlink in fact deletes the file, not merely remove a symbolic link to it.
Thus, when either an NME or Perspective video that points to a "normal" video is deleted, so is the "normal video file".

Modify the delete code to make sure the filename does not point to the wrong kind of video.

The pull request #1398 for video admin does something like this to avoid accidental unintentional domino deletion of normal video files.

[susanodd]

@vanlummelhuizen if you're there, shall I just go ahead and do this? Or do you want to do this? I'm not supposed to keep assigning myself to issues.. :)

[vanlummelhuizen]

@susanodd Please only work on things you discussed with @Woseseltops . So not this issue.

[susanodd]

The changes requested to #1398 asked for the method delete_files to return a Boolean result. In making this change it was on topic to test whether the filename was correct. If it points to the normal video, the filename is erased from the object instead of deleting the file.