handle returned SOR

[mah11]

At present the db tracks whether a given cask is SOR, but doesn’t record explicitly whether a SOR cask is, in fact, sold or returned.

I’m a little hazy on what’s feasible, and what would be intuitive for input & output. Check for follow-up comments.

The most important use case is for the proportion-sold entries under Festival Status.
It’s probably also useful to keep track of what we do with SOR stuff more generally.