[Shuffle] Reduce frequency with which the extension checks if videos have been deleted

[NikkelM]

Currently, the extension always checks if a video has been deleted. However, it does not happen very often that videos get deleted, so this behaviour may add additional overhead to the shuffle whenever it is used. By reducing the amount of times we check for deletion, we can improve the user experience. This could be made dependent on the upload date of the video: Most times, videos are deleted when they are younger, older videos tend to not get deleted as frequently. We would also need a way to keep track of when a video was last checked for deletion - we optimally do not want to have to save a value for this, maybe this could also be done in a way that utilizes the upload date, which we do save.

[NikkelM]

The chosen way to do this will utilize probabilities, with younger videos being more likely checked.