Treat Mailgun metadata like all other ESPs: simple
key-value dict, where values are strings. If you want
to store JSON in metadata, you should serialize and
deserialize it yourself.
* Add smtp-id in unique_args (metadata), to ensure
it shows up in click and open events.
* Add SENDGRID_GENERATE_MESSAGE_ID setting,
default True, to control auto-Message-ID behavior.
* Document it.
