404 for invalid formats

404s are cached in Varnish, 406s aren't. Also arguably an invalid
format is a 404 anyway if it's using a file extension to set the format.
Fix for rendering embedded contacts in govspeak

We have to be explicit about the partial we want so that rendering govspeak in an admin view doesn't try to use the admin/contacts/_contact partial.  Added a test to prove this and made sure the admin_govspeak_helper_test was set up to have an admin prefix so it would fail without the fix.
Pin us to a working version of gds-sso

The version we previously had has dependencies that can't be fulfilled
as they don't exist anymore. It only worked because Bundler keeps a cache.
Fix broken SSO configuration

I broke this in 4c36f1a