Email for saved searches

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/5578

Description (What does it do?)

This PR tweaks the emails sent out for search subscription emails and also activates the search subscription email as a celery task.

Screenshots (if appropriate):

How can this be tested?

1. checkout this branch.
2. go to RC env var settings in heroku and copy down the following into your local settings:
   MAILGUN_SENDER_DOMAIN
   MAILGUN_KEY
   MAILGUN_FROM_EMAIL
   NOTIFICATION_EMAIL_BACKEND
3. restart celery via docker compose restart celery
4. created a saved search subscription that catches learning resources from any provider via the following django shell script (replace user with your own email)

from django.contrib.auth import get_user_model
from learning_resources_search.models import PercolateQuery
from learning_resources_search.api import subscribe_user_to_search_query

User = get_user_model()
migrated_user = User.objects.get(email='[email protected]')
subscribe_user_to_search_query(migrated_user, {"offered_by": ["mitpe", "mitx", "ocw", "see"]}, PercolateQuery.SEARCH_SUBSCRIPTION_TYPE)

6. Change the "created_on" date locally for a few learning resources. I usually manually do it via this django script:

import datetime
delta = datetime.timedelta(days=1)
since = now_in_utc() - delta


resources = LearningResource.objects.filter(published=True)[50:53]

for resource in resources:
	resource.created_on = since  +  datetime.timedelta(hours=1)
	resource.save()

6. manually send out subscription emails for search subscriptions (make sure the email you have locally is an inbox you have access to):

from learning_resources_search.models import PercolateQuery
from learning_resources_search.tasks import send_subscription_emails
send_subscription_emails(PercolateQuery.SEARCH_SUBSCRIPTION_TYPE)