Send siteLogo for Persona popup as data-URI.

I couldn't login on my dev instance due to this bug, so I decided to copy over the fix I applied to Affiliates. It seems to work fine.

siteLogo must be served over HTTPS for the Persona popup, otherwise it
causes an error. If you don’t have HTTPS set up in your local dev 
environment, you can’t log in.

You can get around this by sending the siteLogo as a data-URI, which 
works regardless of protocol.