SendinBlue: additional template/tags improvements

Additional changes related to SendinBlue improvements in #158:

* Support multiple tags in webhooks (closes #162)
* Remove additional outdated template code in backend
* Update integration tests
* Update docs and changelog; note breaking changes as discussed in #161

anymail/backends/sendinblue.py

@@ -83,12 +83,8 @@ class SendinBluePayload(RequestsPayload):
 
     def serialize_data(self):
         """Performs any necessary serialization on self.data, and returns the result."""
-
         if not self.data['headers']:
             del self.data['headers']  # don't send empty headers
-        if self.data.get('templateId') and (self.data.pop('textContent', False) or self.data.pop('htmlContent', False)):
-            self.unsupported_feature("overriding template body content")
-
         return self.serialize_json(self.data)
 
     #

anymail/webhooks/sendinblue.py

@@ -49,10 +49,17 @@ class SendinBlueTrackingWebhookView(AnymailBaseWebhookView):
         except (KeyError, ValueError):
             timestamp = None
 
+        tags = []
         try:
-            tags = [esp_event["tag"]]
+            # If `tags` param set on send, webhook payload includes 'tags' array field.
+            tags = esp_event['tags']
         except KeyError:
-            tags = []
+            try:
+                # If `X-Mailin-Tag` header set on send, webhook payload includes single 'tag' string.
+                # (If header not set, webhook 'tag' will be the template name for template sends.)
+                tags = [esp_event['tag']]
+            except KeyError:
+                pass
 
         try:
             metadata = json.loads(esp_event["X-Mailin-custom"])