Removing Welcome and Returning User notes from posts after a set time

[RGJ (Richard - Communiteq)]

@JammyDodger

However, I think after a certain window of time these notices become relatively meaningless and serve only as extra noise, which I think applies to all sites. It would be useful if these notices could tidy themselves up after their usefulness had passed.

Maybe it would be a good idea to just have .post-notice.old: display: none; shipped in the default CSS so people can always override it if they want to. That makes the change very small and it leverages the existing .old functionality.