feat(api): add referral percentage and only first purchase fields to user model and requests

initialize/migrate/database/02108_user_referral.up.sql

@@ -0,0 +1,7 @@
+ALTER TABLE `u`
+    ADD COLUMN `referral_percentage` TINYINT UNSIGNED NOT NULL DEFAULT 0
+      COMMENT 'Referral Percentage'
+      AFTER `commission`,
+    ADD COLUMN `only_first_purchase` TINYINT(1) NOT NULL DEFAULT 1
+      COMMENT 'Only First Purchase'
+      AFTER `referral_percentage`;