Remove duplicate usernames and make unique

Closes #336

databases/_upgrade_sponsorTimes_23.sql

@@ -0,0 +1,12 @@
+BEGIN TRANSACTION;
+
+DELETE FROM "userNames" WHERE ctid NOT IN (
+    SELECT MIN(ctid) FROM "userNames"
+    GROUP BY "userID"
+);
+
+ALTER TABLE "userNames" ADD UNIQUE("userID");
+
+UPDATE "config" SET value = 23 WHERE key = 'version';
+
+COMMIT;