HG-309 Workaround for issue with being redirected to Special:UserLogin after closing lightbox

It happens from time to time that a user double-taps on the close button in lightbox and he gets redirected to `Special:UserLogin` if it happens top navigation bar is visible on the page.

Below is a simple workaround which hides the top navigation bar every time somebody opens lightbox. It's an idea which doesn't need to be merged :japanese_ogre: 
I think if we do implement this hack, we should at least keep the naming consistent. Elsewhere in the code, the top nav is referred to as the site header, so I think "SiteNav" and "TopNav" should be "SiteHeader" instead.