Added endpoint to get the letter preview from the template preview app

Is will allow the admin application to call into the api instead of
making the call itself. This will allow the api to make decision for
precompiled pdf without having to update the admin app.

- Added new endpoint
- Added tests for the endpoint

app/template/rest.py

@@ -1,10 +1,11 @@
 from flask import (
     Blueprint,
+    current_app,
     jsonify,
-    request,
-    current_app
+    request
 )
 
+from app.dao.notifications_dao import get_notification_by_id
 from app.dao.templates_dao import (
     dao_update_template,
     dao_create_template,
@@ -12,8 +13,8 @@ from app.dao.templates_dao import (
     dao_get_template_by_id_and_service_id,
     dao_get_all_templates_for_service,
     dao_get_template_versions,
-    dao_update_template_reply_to
-)
+    dao_update_template_reply_to,
+    dao_get_template_by_id)
 from notifications_utils.template import SMSMessageTemplate
 from app.dao.services_dao import dao_fetch_service_by_id
 from app.models import SMS_TYPE
@@ -179,3 +180,43 @@ def redact_template(template, data):
     if not template.redact_personalisation:
         dao_redact_template(template, data['created_by'])
     return 'null', 200
+
+
+@template_blueprint.route('/<uuid:template_id>/pdf-preview/<uuid:notification_id>/<file_type>', methods=['GET'])
+def preview_letter_template_by_notification_id(service_id, template_id, notification_id, file_type):
+
+    if file_type not in ('pdf', 'png'):
+        raise InvalidRequest({'content': ["file_type must be pdf or png"]}, status_code=404)
+
+    page = request.args.get('page')
+
+    template = dao_get_template_by_id(template_id)
+
+    template_for_letter_print = {
+        "subject": template.subject,
+        "content": template.content
+    }
+
+    notification = get_notification_by_id(notification_id)
+
+    service = dao_fetch_service_by_id(service_id)
+
+    data = {
+        'letter_contact_block': notification.reply_to_text,
+        'template': template_for_letter_print,
+        'values': notification.personalisation,
+        'dvla_org_id': service.dvla_organisation_id,
+    }
+
+    from requests import (post as requests_post)
+    resp = requests_post(
+        '{}/preview.{}{}'.format(
+            current_app.config['TEMPLATE_PREVIEW_API_HOST'],
+            file_type,
+            '?page={}'.format(page) if page else ''
+        ),
+        json=data,
+        headers={'Authorization': 'Token {}'.format(current_app.config['TEMPLATE_PREVIEW_API_KEY'])}
+    )
+
+    return resp.content, resp.status_code, resp.headers.items()