Track Sendinblue API updates:
* Multiple tags are now supported
* When using a template, display name is now supported on 'to', 'bcc', 'cc' and 'replyTo'
* Templates now support overriding 'from_email' and 'subject'
* Templates no longer require separate API endpoint
* 'merge_global_data' can be used without templates
Django allows setting the reply address with either message.reply_to
or message.extra_headers["Reply-To"]. If both are supplied, the extra
headers version takes precedence. (See EmailMessage.message().)
Several Anymail backends had duplicate logic to handle conflicting
properties. Move that logic into the base Payload.
(Also prepares for common handling of extra_headers['From'], later.)
Related changes:
* Use CaseInsensitiveDict for processing extra_headers.
This is potentially a breaking change, but any code that was trying
to send multiple headers differing only in case was likely already
broken. (Email header field names are case-insensitive, per RFC-822.)
* Handle CaseInsensitiveDict in RequestsPayload.serialize_json().
(Several backends had duplicate code for handling this, too.)
* Fixes SparkPost backend, which had been incorrectly treating
message.reply_to and message.extra_headers['Reply-To'] differently.
Add support for sending transactional email through SendinBlue. (Thanks to @RignonNoel.)
Partially implements #84. (Tracking webhooks will be a separate PR. SendinBlue doesn't support inbound handling.)
