use letter branding object instead of letter_logo_filename

This is part of migrating away from dvla organisations. This only
affects in-app previews and not actual letters printed by the api.

app/models/service.py

@@ -29,7 +29,6 @@ class Service():
         'inbound_api',
         'letter_branding',
         'letter_contact_block',
-        'letter_logo_filename',
         'message_limit',
         'name',
         'organisation_type',

app/template_previews.py

@@ -11,7 +11,7 @@ class TemplatePreview:
             'letter_contact_block': template.get('reply_to_text', ''),
             'template': template,
             'values': values,
-            'filename': current_service.letter_logo_filename,
+            'filename': current_service.letter_branding and current_service.letter_branding['filename']
         }
         resp = requests.post(
             '{}/preview.{}{}'.format(