Send out transactional emails to users 3 days after Webmaker install

[secretrobotron]

We need to send emails to users 3 days after they install and sign up to use Webmaker app.

@jbuck's advice was to use a worker process here, in login.webmaker.org, to wake up each day (or hour?) and send messages through hatchet to any 3-day-old users.

@hpauljohnson and @kevzawacki will have content for the email which will be sent out through sawmill via webmaker-mailroom.

cc @hpauljohnson @jbuck

[thisandagain]

I think I'd prefer to spin-up a new "scheduler" service that uses hatchet to push jobs into the SQS queue. It seems odd to piggyback that functionality on-top of login. Thoughts @simonwex @jbuck ?

[jbuck]

Yeah, I think Bobby and I discussed this idea before we had that discussion. Lets go with the scheduler idea, it's better