Fix #3866 - Use RP branding on login popup in main site.

Only includes the `siteLogo` if the page is being served over SSL; local development sign-in fails if you don't do this since it's not HTTPS. I considered using a data URI, but couldn't figure out a way to include it that would stay updated with the image file and wouldn't require something silly that's currently beyond my skills, like a (whatever the equivalent to context processors from Django are in express-land) that adds the data URI to the template context for embedding in `layout.ejs`.

Here's a preview of what it looks like for me (using a data URI temporarily to make the image display):
![screen shot 2014-03-23 at 2 43 16 pm](https://f.cloud.github.com/assets/193106/2494173/0755d066-b2bb-11e3-810b-ef4c75dd0845.png)