Add NEXTAUTH_URL to app.json to address heroku deploy failure (#3091)

app.json

@@ -18,7 +18,11 @@
       "description": "Application Key for symmetric encryption and decryption. Must be 32 bytes for AES256 encryption algorithm.",
       "value": "secret"
     },
-    "NEXTAUTH_SECRET": "secret"
+    "NEXTAUTH_SECRET": "secret",
+    "NEXTAUTH_URL": {
+      "description": "Replace HEROKU_APP_NAME with the name given to your app",
+      "value": "https://HEROKU_APP_NAME.herokuapp.com"
+    }
   },
   "scripts": {
     "postdeploy": "cd packages/prisma && npx prisma migrate deploy"