Look in all parts of a letter template to find placeholders

Text messages have placeholders in their body.

Emails have them in their subject line too.

Letters have them in their body, subject line and contact block.

We were only looking in the the body and subject when processing a job,
therefore the thing assembling the letter was not looking in all the
CSV columns it needed to, because it hadn’t been told about any
placeholders in the contact block.

Fixing this means always making sure we use the correct `Template`
instance for the type of template we’re dealing with. Which we were
already doing in a different part of the codebase. So it makes sense to
reuse that.

Turns out we fixed the same bug for email subjects over 3 years ago:
3ed97151ee

app/celery/tasks.py

@@ -8,10 +8,6 @@ from notifications_utils.recipients import (
     RecipientCSV
 )
 from notifications_utils.statsd_decorators import statsd
-from notifications_utils.template import (
-    SMSMessageTemplate,
-    WithSubjectTemplate,
-)
 from notifications_utils.timezones import convert_utc_to_bst
 from requests import (
     HTTPError,
@@ -128,9 +124,8 @@ def job_complete(job, resumed=False, start=None):
 
 def get_recipient_csv_and_template_and_sender_id(job):
     db_template = dao_get_template_by_id(job.template_id, job.template_version)
+    template = db_template._as_utils_template()
 
-    TemplateClass = get_template_class(db_template.template_type)
-    template = TemplateClass(db_template.__dict__)
     contents, meta_data = s3.get_job_and_metadata_from_s3(service_id=str(job.service_id), job_id=str(job.id))
     recipient_csv = RecipientCSV(file_data=contents,
                                  template_type=template.template_type,
@@ -454,15 +449,6 @@ def handle_exception(task, notification, notification_id, exc):
             current_app.logger.error('Max retry failed' + retry_msg)
 
 
-def get_template_class(template_type):
-    if template_type == SMS_TYPE:
-        return SMSMessageTemplate
-    elif template_type in (EMAIL_TYPE, LETTER_TYPE):
-        # since we don't need rendering capabilities (we only need to extract placeholders) both email and letter can
-        # use the same base template
-        return WithSubjectTemplate
-
-
 @notify_celery.task(bind=True, name='update-letter-notifications-statuses')
 @statsd(namespace="tasks")
 def update_letter_notifications_statuses(self, filename):