Allowed constraintName in schema column spec

refs 0ba3d6df49

- this is used to indicate the name of the foreign key constraint and so
  we should let it through the schema checks

ghost/core/test/unit/server/data/schema/schema.test.js

@@ -29,6 +29,7 @@ const VALID_KEYS = {
         'validations',
         'defaultTo',
         'references',
+        'constraintName',
         'cascadeDelete',
         'setNullDelete',
         'index'