Mailgun: Support stored templates

Add support for Mailgun's new template option

Fixes #155

anymail/backends/mailgun.py

@@ -253,8 +253,8 @@ class MailgunPayload(RequestsPayload):
     def set_track_opens(self, track_opens):
         self.data["o:tracking-opens"] = "yes" if track_opens else "no"
 
-    # template_id: Mailgun doesn't offer stored templates.
-    # (The message body and other fields *are* the template content.)
+    def set_template_id(self, template_id):
+        self.data["template"] = template_id
 
     def set_merge_data(self, merge_data):
         # Processed at serialization time (to allow merging global data)

docs/esps/mailgun.rst

@@ -233,14 +233,10 @@ Limitations and quirks
 
 .. _mailgun-templates:
 
-Batch sending/merge and ESP templates
+Batch sending/merge
 -------------------------------------
 
-Mailgun does not offer :ref:`ESP stored templates <esp-stored-templates>`,
-so Anymail's :attr:`~anymail.message.AnymailMessage.template_id` message
-attribute is not supported with the Mailgun backend.
-
-Mailgun *does* support :ref:`batch sending <batch-send>` with per-recipient
+Mailgun supports :ref:`batch sending <batch-send>` with per-recipient
 merge data. You can refer to Mailgun "recipient variables" in your
 message subject and body, and supply the values with Anymail's
 normalized :attr:`~anymail.message.AnymailMessage.merge_data`

tests/test_mailgun_backend.py

@@ -448,7 +448,11 @@ class MailgunBackendAnymailFeatureTests(MailgunBackendMockAPITestCase):
         self.assertEqual(data['o:tracking-opens'], 'no')
         self.assertEqual(data['o:tracking-clicks'], 'yes')
 
-    # template_id: Mailgun doesn't support stored templates
+    def test_template_id(self):
+        self.message.template_id = "welcome_template"
+        self.message.send()
+        data = self.get_api_call_data()
+        self.assertEqual(data['template'], "welcome_template")
 
     def test_merge_data(self):
         self.message.to = ['alice@example.com', 'Bob <bob@example.com>']