Return rendered HTML when previewing a template

If you’re trying to show what a Notify email will look like in your
caseworking system all the API gives you at the moment is raw markdown
(with the placeholders replaced).

This isn’t that useful if your caseworkers have no idea what markdown
is. If we also give teams the HTML then they can embed this in their
systems, and the people using those systems will be able to see how
headings, bulleted lists, etc. look.

app/v2/template/template_schemas.py

@@ -1,5 +1,6 @@
 from app.models import SMS_TYPE, TEMPLATE_TYPES
 from app.schema_validation.definitions import uuid, personalisation
+from app.utils import get_html_email_body_from_template
 
 
 get_template_by_id_request = {
@@ -79,6 +80,7 @@ def create_post_template_preview_response(template, template_object):
         "type": template.template_type,
         "version": template.version,
         "body": str(template_object),
+        "html": get_html_email_body_from_template(template_object),
         "subject": subject,
         "postage": template.postage
     }