Fixes for tests broken by changes

- corrects target page for set_email_branding to
  new preview step instead of itself
- removed check for helper method being called in
  email page test
- updates expected result for test of global
  headers to include changes to `frame-src`
- updates navigation config with brand preview page

tests/app/main/views/test_email_preview.py

@@ -10,11 +10,9 @@ from flask import url_for
 )
 def test_renders(client, mocker, query_args, result):
 
-    mock_convert_to_boolean = mocker.patch('app.main.views.index.convert_to_boolean')
     mocker.patch('app.main.views.index.HTMLEmailTemplate.__str__', return_value='rendered')
 
     response = client.get(url_for('main.email_template', **query_args))
 
     assert response.status_code == 200
     assert response.get_data(as_text=True) == 'rendered'
-    mock_convert_to_boolean.assert_called_once_with(result)