Revert "3054592: Add support for postmark templates."
This MR adds functionality to Postmark module that allows users to use Postmark templates.
- Adds a new plugin 'postmark_template_mail' that facilitates this new method
- Adapts PostmarkHandler to receive template vars and a template ID in the message payload, and uses the template API endpoint if it is a template mail send. An alter hook has also been added to amend template vars by another module if required, e.g for adding vars that are consistent on every email, like company name or header banner img src.
For an improved developer experience the Postmark configuration form has been adapted to the following ends:
- Test email now has its own submit button and validation handler.
- Postmark mail plugin may now be chosen for the test email...
- If 'Postmark template mailer' plugin is chosen, a field is revealed that allows you to test variables to populate on the template on Postmark's end. The field requires a JSON object which is validated as part of the test email submit's validation handler.
- Test values are now stored in state for ease of repeated testing.
This reverts commit 95f99e80.
Closes #3054592