Bump sequelize from 5.12.2 to 5.15.1

Bumps [sequelize](https://github.com/sequelize/sequelize) from 5.12.2 to 5.15.1.
- [Release notes](https://github.com/sequelize/sequelize/releases)
- [Commits](sequelize/sequelize@v5.12.2...v5.15.1)

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

Author: dependabot[bot]

package-lock.json

@@ -18,7 +18,7 @@
     "express": "^4.17.1",
     "express-session": "^1.15.6",
     "mysql2": "^1.6.5",
-    "sequelize": "^5.12.2"
+    "sequelize": "^5.15.1"
   },
   "devDependencies": {
     "eslint": "^6.2.1",