Support rollback of deployment updates

[stuartwdouglas]

We should support rolling back deployment updates. We need to be careful with schema compatibility though, if this new update has changed the schema the old deployment may not be considered compatible and more (e.g. a new field has been added, and the removal via rollback would be considered an incompatible change).

#2837 could be used as a building block to implement this, if we know exactly what versions of a given deployment anther deployment was compiled against we could use this to figure out which existing deployments are compatible with the old version, and which also need to be rolled back.

We also need to think about deployment sets, e.g. the PFI is multiple deployments, but they are all released from the same git commit, and I don't know if it would make sense to roll back some of the modules but not others.

There is a quite a lot of complexity here, but we probably want at least basic support for production usage.