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.

tests/app/test_template_previews.py

@@ -102,3 +102,20 @@ def test_page_count_unpacks_from_json_response(
 
     assert partial_call({'template_type': 'letter'}) == 99
     mock_template_preview.assert_called_once_with(*expected_template_preview_args)
+
+
+def test_from_example_template_makes_request(mocker):
+    request_mock = mocker.patch('app.template_previews.requests.post')
+    template = {}
+    filename = 'geo'
+
+    TemplatePreview.from_example_template(template, filename)
+
+    request_mock.assert_called_once_with(
+        'http://localhost:9999/preview.png',
+        headers={'Authorization': 'Token my-secret-key'},
+        json={'values': None,
+              'template': template,
+              'filename': filename,
+              'letter_contact_block': None}
+    )