video updated field doesn't get updated when tags, relatedurls, etc change

[willkg]

If you add a tag, related url or something like that to a Video, it doesn't update the updated field.

This might be a problem with sitemaps where we're stating that the updated field represents when the video was last updated. I don't think this affects anything else, though, but it's probably worth fixing.

The easy fix might be to add post-save hooks to things and have those update the updated field.