#5392616 Titan: Show control panel in an iframe

Changes proposed in this Pull Request

Instead of showing an auto-login link, let’s embed Titan’s control panel in an iframe using their special API.

Testing instructions

Make sure you’ve applied D54806-code on your sandbox.
You need to enable Store Sandbox as well – the production control panel iframe has additional security checks which will prevent it from being displayed (CSP et al.)

On a domain with an Titan email, go to example.com -> Email. With the feature flag disabled, it should look like this:

With the feature flag enabled (it’s enabled by default on development and wpcalypso):

Known issues

There seems to be a small visual issue with the back button within the control panel:

That should be easily fixed by the Titan team.