Updates in site titles and headers

With the recent change in translateable site titles and headers, we had to change the way they are rendered on page. Here's what's changed:

site.name  page.site_title

The {{site.name}} property on site object is now deprecated and should no longer be used. Use {{page.site_title}} instead. It renders the value entered in "Site title" input field in language create and edit form.

We'll keep the old {{site.name}} property for long time so nothing will break. Though, by using it, your site title is not translateable.