Batch email sending

[yanchuk]

https://api-docs.mailtrap.io/docs/mailtrap-api-docs/f96000c8de409-batch-send-email-including-templates
https://api-docs.mailtrap.io/docs/mailtrap-api-docs/c323263dc45c5-batch-send-email-including-templates
https://api-docs.mailtrap.io/docs/mailtrap-api-docs/48b526aa487b1-batch-send-email-including-templates
2 streams + sandbox

[vitalii-t]

#38