DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
Add columns for each custom notification level, defaulting to null. Read from those columns if non-null, otherwise fall back to the serialized column. Writing will write to the new column if `events` isn't manually set.