We have gained:
* Haskell-side validation of schema changes before their execution
* Report of results of migration process
* Handling of old deployments
However:
* The validation code hasn't been tested yet at all
* Most of the migration list hasn't been applied at all yet
* Adding lists of entities from a model file is NOT VALIDATED!!! It's totally
possible to implement, just need to catch all the small details right
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.
