Show letter branding preview when selecting a brand

This adds a preview pane which is visible when updating a letter brand.

If JavaScript is enabled, the preview pane shows on the set-letter-branding
page, and submitting the form saves updates the letter brand for a service
immediately. If Javascript is not enabled, there is a separate 'Preview email
branding' page which shows a preview of the brand and has a 'Save' button on it.

app/template_previews.py

@@ -24,6 +24,21 @@ class TemplatePreview:
         )
         return (resp.content, resp.status_code, resp.headers.items())
 
+    @classmethod
+    def from_example_template(cls, template, filename):
+        data = {
+            'letter_contact_block': template.get('reply_to_text'),
+            'template': template,
+            'values': None,
+            'filename': filename
+        }
+        resp = requests.post(
+            '{}/preview.png'.format(current_app.config['TEMPLATE_PREVIEW_API_HOST']),
+            json=data,
+            headers={'Authorization': 'Token {}'.format(current_app.config['TEMPLATE_PREVIEW_API_KEY'])}
+        )
+        return (resp.content, resp.status_code, resp.headers.items())
+
     @classmethod
     def from_utils_template(cls, template, filetype, page=None):
         return cls.from_database_object(