Database view is broken with mongo-express

[apkar]

mongo-express admin web interface is broken with

    at /app/server/node_modules/mongo-express/lib/routes/database.js:40:49
    at handleCallback (/app/server/node_modules/mongodb/lib/utils.js:95:56)
    at /app/server/node_modules/mongodb/lib/db.js:313:5
    at /app/server/node_modules/mongodb-core/lib/connection/pool.js:455:18
    at process.internalTickCallback (internal/process/next_tick.js:70:11)

It is failing in this line of the code, where it is trying to show numExtents of dbStats.

https://docs.mongodb.com/manual/reference/command/dbStats/#dbStats.numExtents

Blocking #29