Some shreddit URLs are not the same as their old reddit counterparts; "Open in old/new Reddit" breaks #1029
Description
This is going to be a pain to work around - we'll have to store a mapping of known URL differences between old Reddit and Shreddit in a way that lets us transform one to the other easily, and ideally do it in such a way that we can add support for new frontends later in case we ever need to do translation between e.g. shreddit and modmail for some reason (or god forbid another redesign comes along).