Allow linking to older versions of a series

[kuba-moo]

Add support for automation to link an older version of a series to a newer one.

[pevik]

I would rename the issue to "Link older versions of series together" (allow suggest one would choose and administrate this, but just some quite well working mechanism would be good enough). I suppose it would be based just on the patch subject. But it would be great if it linked also patches send by different user.

I suppose this feature would require modifying database layout, right?
Here are at least DB installation details (layout does not look to be described)
https://github.com/getpatchwork/patchwork/blob/main/docs/development/installation.rst
https://patchwork.readthedocs.io/en/latest/deployment/installation.html#database