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/commands.py

@@ -17,7 +17,7 @@ from notifications_utils.statsd_decorators import statsd
 
 from app import db, DATETIME_FORMAT, encryption
 from app.aws import s3
-from app.celery.tasks import record_daily_sorted_counts, get_template_class, process_row
+from app.celery.tasks import record_daily_sorted_counts, process_row
 from app.celery.nightly_tasks import send_total_sent_notifications_to_performance_platform
 from app.celery.service_callback_tasks import send_delivery_status_to_service
 from app.celery.letters_pdf_tasks import create_letters_pdf
@@ -889,8 +889,7 @@ def process_row_from_job(job_id, job_row_number):
     job = dao_get_job_by_id(job_id)
     db_template = dao_get_template_by_id(job.template_id, job.template_version)
 
-    TemplateClass = get_template_class(db_template.template_type)
-    template = TemplateClass(db_template.__dict__)
+    template = db_template._as_utils_template()
 
     for row in RecipientCSV(
             s3.get_job_from_s3(str(job.service_id), str(job.id)),