Use deliver_later for email sending #2038
This PR builds on #2037 and configures email sending as a background task.
Note that this depends on having the setup for delayed_job (#2037) already merged, along with having workers running in production (openstreetmap/chef#201), before this can be deployed - so I'm marking it as WIP until those prerequisites are met.
Looks reasonable but I'll want to be a bit careful deploying it so I can monitor things and make sure everything is working as expected...
I'm wondering what to do about job queueing in development mode though - maybe use the inline adaptor so that things execute immediately without everybody having to run the daemon?