Move the email digests template into Python/Jinja2

Move the email digests template from Mailchimp into Python/Jinja2.
This involves:

1. Creating a new Jinja2 template for the email at
   `lms/templates/email/instructor_email_digest/` based on the template
   that we previously had in Mailchimp. This is basically exactly the
   same as the template that we have in Mailchimp except that I fixed
   the formatting/indentation and I replaced Mailchimp's template tags
   with Jinja2 ones.

2. Adding a new `MailchimpService.send()` method (instead of the
   existing `MailchimpService.send_template()`) that uses Jinja2 to
   render the new template and then calls the Mailchimp API passing the
   rendered text to send the email.

3. Adding a new Celery task `send()` (instead of the existing
   `send_template()` task) that calls `MailchimpService.send()`
   instead of `send_template()`.

4. Changing `DigestService` to call the new `send()` task instead of the
   old `send_template()` task when sending digest emails.

I also added one image file that was previously hosted on Mailchimp into
our app's static files. The new email template references this file.

The original `MailchimpService.send_template()` method and
`send_template()` Celery task are still present: these will be needed in
case there are any `send_template` messages remaining on the queue. A
follow-up commit will remove these methods and their tests once the
queue has been drained.

lms/services/digest.py

@@ -19,7 +19,7 @@
 from lms.services.email_unsubscribe import EmailUnsubscribeService
 from lms.services.h_api import HAPI
 from lms.services.mailchimp import EmailRecipient, EmailSender
-from lms.tasks.mailchimp import send_template
+from lms.tasks.mailchimp import send
 
 LOG = logging.getLogger(__name__)
 
@@ -75,9 +75,9 @@ def send_instructor_email_digests(  # pylint:disable=too-many-arguments
             else:
                 task_done_key = None
 
-            send_template.delay(
+            send.delay(
                 task_done_key=task_done_key,
-                template_name="instructor-email-digest",
+                template="lms:templates/email/instructor_email_digest/",
                 sender=asdict(self._sender),
                 recipient=asdict(EmailRecipient(to_email, unified_user.display_name)),
                 template_vars=digest,

lms/services/mailchimp.py

@@ -1,8 +1,10 @@
 import logging
 from dataclasses import dataclass
+from pathlib import Path
 from typing import Optional
 
 import mailchimp_transactional
+from pyramid.renderers import render
 from sqlalchemy import select
 
 from lms.models import TaskDone
@@ -40,6 +42,68 @@ def __init__(self, db, api_key):
         self.db = db
         self.mailchimp_client = mailchimp_transactional.Client(api_key)
 
+    def send(  # pylint:disable=too-many-arguments
+        self,
+        template: str,
+        sender: EmailSender,
+        recipient: EmailRecipient,
+        template_vars: dict,
+        unsubscribe_url: Optional[str] = None,
+        task_done_key: Optional[str] = None,
+    ):
+        """
+        Send an email using Mailchimp Transactional's API.
+
+        https://mailchimp.com/developer/transactional/api/messages/send-new-message/
+        """
+
+        if task_done_key:
+            if self.db.execute(
+                select(TaskDone).filter_by(key=task_done_key)
+            ).one_or_none():
+                LOG.info("Not sending duplicate email %s", task_done_key)
+                return
+
+        headers = {}
+
+        if unsubscribe_url:
+            template_vars["unsubscribe_url"] = unsubscribe_url
+            headers["List-Unsubscribe"] = unsubscribe_url
+
+        subject = render(str(template / Path("subject.jinja2")), template_vars)
+
+        params = {
+            "message": {
+                "subject": subject,
+                "html": "...",  # For logging only, will be replaced below.
+                "subaccount": sender.subaccount,
+                "from_email": sender.email,
+                "from_name": sender.name,
+                "to": [{"email": recipient.email, "name": recipient.name}],
+                "track_opens": True,
+                "track_clicks": True,
+                "auto_text": True,
+                "headers": headers,
+            },
+            "async": True,
+        }
+
+        LOG.info("mailchimp_client.send(%r)", params)
+
+        # The HTML body might be long so add it to the params *after* logging.
+        params["message"]["html"] = render(
+            str(template / Path("body.html.jinja2")), template_vars
+        )
+
+        try:
+            self.mailchimp_client.messages.send(params)
+        except Exception as exc:
+            raise MailchimpError() from exc
+
+        if task_done_key:
+            # Record the email send in the DB to avoid sending duplicates.
+            self.db.add(TaskDone(key=task_done_key))
+
     def send_template(  # pylint:disable=too-many-arguments
         self,
         template_name: str,

lms/tasks/mailchimp.py

@@ -4,6 +4,26 @@
 from lms.tasks.celery import app
 
 
+@app.task(
+    acks_late=True,
+    autoretry_for=(Exception,),
+    max_retries=2,
+    retry_backoff=3600,
+    retry_backoff_max=7200,
+)
+def send(*, sender, recipient, **kwargs) -> None:
+    """Send an email using Mailchimp's API."""
+
+    sender = EmailSender(**sender)
+    recipient = EmailRecipient(**recipient)
+
+    with app.request_context() as request:  # pylint:disable=no-member
+        mailchimp_service = request.find_service(name="mailchimp")
+        print(request.environ)
+        with request.tm:
+            mailchimp_service.send(sender=sender, recipient=recipient, **kwargs)
+
+
 @app.task(
     acks_late=True,
     autoretry_for=(Exception,),