Add an option to add a prefix to every ID of rendered output to avoid conflicts across comments and issue

We noticed this is hard to implement, because we want internal links to still work, and just adding a prefix to every ID does not take that into account.

We later noticed that what we actually want to solve the comment use case, is a custom toplevel scope, which we can easily implement with a custom named directory. So... scopes save the day for once?

Will be useful for comments on web, since a single author can make multiple comments, so prefixing by usernme won't be enough.

For topic pages, we can just prefix by username, and that is already currently done.