[Feature Request] share link base url config

Description My use case for this is that I access paperless via a host like docs.private.example.com. My reverse proxy limits access to that host to clients on my lan. I expose shares (like paperless share links) via a public facing host like docs.share.example.com. I don't access paperless via docs.share.example.com because that host is limited to paths like /share/* (to limit the surface area I'm exposing publicly) so I can't use the default share link generation behavior to get me working, public, share links in one click. Of course I can simply generate the share link and then replace "private" with "share", but that sucks, and my wife is not going to do that. I'd like to add a config like PAPERLESS_SHARE_LINK_BASE_URL that I can use to set the share link base url to docs.share.example.com instead of the docs.public.example.com that I use to access the instance. Maybe there are other folks with a similar desire? I think this is a p…