Bump sequelize from 5.15.1 to 5.21.3

Bumps [sequelize](https://github.com/sequelize/sequelize) from 5.15.1 to 5.21.3.
- [Release notes](https://github.com/sequelize/sequelize/releases)
- [Commits](https://github.com/sequelize/sequelize/compare/v5.15.1...v5.21.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -79,7 +79,7 @@
     "passwords": "^1.3.1",
     "pg": "^7.17.1",
     "redis": "^2.8.0",
-    "sequelize": "^5.15.1",
+    "sequelize": "^5.21.3",
     "smooth-scroll": "https://github.com/cferdinandi/smooth-scroll#5.3.7",
     "statsy": "~0.2.0",
     "stripe": "^7.1.0",