Enable optimistic locking for resources; DDR-2050.

When upgrading an application to use this version of ddr-core, a new ddr-core migration will need to be installed and run and then the lock_version column of all resources will need to be updated to 0. (Technically, the column needs to be updated for all resources which have nil as the current lock_version column value ... but that is presumably all of them.) A command like this can probably be used to do so -- Valkyrie::Persistence::Postgres::ORM::Resource.where(lock_version: nil).update_all(lock_version: 0) -- or, I guess, a direct psql command.