All the migrations have been updated to use BIGINTs for ID fields in the DB, but ActiveRecord needs to be told to treat those values as BIGINT as well. This PR does that.
* Add foreign key constraint to column `account` in `account_moderation_notes`
* Change account_id and target_account_id to non-nullable in account_moderation_notes
* Add dependent: :destroy to account and target_account in account_moderation_notes