Revision woes

There is a lot of complexity and consequent room for user error in the current revisioning process:

- The unadorned PDF (`{anthid}.pdf`) is always replaced so as to work with our redirect rules
- For the first revision, `{anthid}v1.pdf` has to be created
- We have to update checksums on both the `` and `` tags
- It's also strange how Revision 1 gets a "v2" extension

I wonder if we should get rid of `` and `` in favor of generic `` tags with a revision attribute. It would also be nice to avoid renaming at all. This would break the ".pdf" URL shortcut, but maybe that doesn't matter—users would just have to visit the paper page to get the latest revision.

> _Originally posted by @mjpost in https://github.com/acl-org/acl-anthology/pull/903#issuecomment-655524448_