Offload emails to background worker

[dirn]

There are a several actions that can result in emails being sent. Currently these are all part of Flask-Security, but eventually we'd like to automate some additional ones (#220, #245). If an error occurs sending an email, the user sees an error page. This shouldn't be the case. Instead we should add the emails to a queue and process them through a separate worker. We'd like to use RQ for this.

Since all of the current uses are inside Flask-Security, we can use its send_mail_task decorator to accomplish this.

In addition to adding RQ to the code base, we'll need to add Redis and a worker to the Docker Compose file.

[dirn]

This may not be as straightforward as originally thought. See mattupstate/overholt#33.