Add script that let's you manually run cron form the command line

As mentioned in #2344, #2126.

To use:

    node ./src/server.js --cron

You can also specify the current hour to run it manually for other users. `--cron-hour` is the hour in the server's timezone.

    node ./src/server.js --cron --cron-hour 14

I think it may need a database migration to add user.cronTime. Not sure how defaults work and all that.

Once vetted, you would probably want this setup with  [Heroku Scheduler](https://devcenter.heroku.com/articles/scheduler).