Support Postmark batch send and per-recipient merge_data #122
Description
For most ESPs, Anymail supports batch sending. If message.merge_data is set, a separate copy of the message is sent to each recipient in the to list, potentially with per-recipient customizations (e.g., when combined with template_id).
The original release of Postmark's template APIs didn't support batch sending, and Anymail currently (v4.1) raises an unsupported feature error in the Postmark backend if a message's merge_data is set.
Postmark appears to have quietly added /email/batchWithTemplates since the original release. Anymail should use that API (and /email/batch for batch sends without a template) to implement batch sending for Postmark.