Refuse dragged items as described in #165

Very simple change to refuse dragged items in `PBWebController`'s `WebView`. Although #165 only mentions the diff view, it doesn't look like any of those managed by `PBWebController` should be accepting drags so this covers all of them. If that's not correct, it's easy to move the same `-webView:dragDestinationActionMaskForDraggingInfo:` into the individual view classes, of course.