My parser doesn't support default=, and I think it's safe to assume nobody is
running an instance whose DB schema version is one of those first 5 where
default= exists, so I'm remvoing it from the original 2016 model file. The
unset-default migrations remain, and I checked in `psql` that PostgreSQL
accepts the operation on columns that don't have a default value. If it turns
out to be a problem, I can replace those migrations with no-op ones.
Until now the list of DB migration actions was incomplete, containing only
changes made since I added the migration system itself. It now contains the
2016-08-04 model, and then every change made since then.
IMPORTANT: The 2016-08-04 instance doesn't have a schema version entity at all,
so it is assigned version 0, while the actual version of its schema is 1. I'm
going to patch persistent-migration to allow it to be 1, making the migration
path smooth.
