Migrate keywords column

[cmars]

#2 changes the contents of the keywords field/column. This is a reminder to provide a migration for updating existing databases accordingly with the next release.

[sfllaw]

I'd like to get a new release with this code in it, what is your preferred mechanism for helping people update their databases? Of course, this only applies to people running Postgres. I believe that the Mongo version already had this behaviour before.

[sfllaw]

@cmars Any ideas on how you want to approach this? One of our engineers is looking into a PL/pgSQL that can be run as a data migration.

[cmars]

@sfllaw That'd be helpful. Happy to provide such a helper script in the next release.