core: use saslprep to limit usernames

packages/hydrooj/package.json

@@ -43,6 +43,7 @@
         "p-queue": "^7.3.0",
         "path-to-regexp": "^6.2.1",
         "require-resolve-hook": "^1.1.0",
+        "saslprep": "^1.0.3",
         "schemastery": "^3.5.4",
         "semver": "^7.3.7",
         "serialize-javascript": "^6.0.0",
@@ -64,6 +65,7 @@
         "@types/mongodb": "^3.6.20",
         "@types/nodemailer": "^6.4.5",
         "@types/notp": "^2.0.2",
+        "@types/saslprep": "^1.0.1",
         "@types/semver": "^7.3.12",
         "@types/serialize-javascript": "^5.0.2",
         "@types/superagent": "^4.1.15",