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

@@ -51,7 +51,7 @@
     "pg": "^7.17.1",
     "playmusic": "^2.3.0",
     "sass-loader": "^7.0.1",
-    "sequelize": "^5.15.1",
+    "sequelize": "^5.21.3",
     "spotify-web-api-node": "^4.0.0",
     "superagent": "^5.0.6",
     "vue": "^2.6.11",