Make letter preview show up on job page

It wasn’t being called with the URL, so the contents of the letter were
just showing up as plain text on the page.

app/main/views/jobs.py

@@ -118,6 +118,13 @@ def view_job(service_id, job_id):
                 version=job['template_version']
             )['data'],
             current_service,
+            letter_preview_url=url_for(
+                '.view_template_version_preview',
+                service_id=service_id,
+                template_id=job['template'],
+                version=job['template_version'],
+                filetype='png',
+            ),
         ),
         status=request.args.get('status', ''),
         updates_url=url_for(