Removes any checks for branding_type from tests
for set_email_branding page.
Updates tests for email preview page (usually
iframed) so non-default brand_type is got from the
email_branding model instead of a GET param.
- 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
