R1:7f2c90fbd12b

Prepare for multiple mailers of the same type

Summary:
Depends on D19000. Ref T13053. Ref T12677. Currently, most mailers are configured with a bunch of `<mailer>.setting-name` global config options.

This means that you can't configure two different SMTP servers, which is a reasonable thing to want to do in the brave new world of mail failover.

It also means you can't configure two Mailgun accounts or two SES accounts. Although this might seem a little silly, we've had more service disruptions because…