strong-migrations warns about dangerous operation when adding foreign key

[rchoquet]

When using safe-pg-migrations together with https://github.com/ankane/strong_migrations, the latter warns about add_foreign_key being a dangerous operation.

safe-pg-migrations makes it a safe operation by doing it in two steps (adding as not valid / validate), adding a foreign key could be marked as safe (as done with add_index) by safe-pg-migrations