Pushing user invitation email generation to the background

[jordanh]

Issue - Enhancement

We currently render email invitations within the inviteTeamMembers mutation, blocking the return of the graphql call – causing the client to wait while the server does its work.

We should instead enqueue this work and run it in a background task. There are a few options:

1. Use node-threads-a-gogo to spawn a new thread
2. Spawn a subprocess for this work – a step toward an email micro-service perhaps (perhaps using child-process-promise
3. Use a new/light rethinkdb-backed job queue like node-rethink-job-queue
4. Use a tried-and-true redis-backed job queue like bull
5. Some other option?

3 looks like the most obvious option, any other opinions?

@mattkrick what chu think?