Issue #2067915 by eatings, kjay, bserem: Restore html attributes to maintenance-page.tpl.php.

PR for https://github.com/backdrop/backdrop-issues/issues/200
We're not using twig templates in Backdrop, so the syntax here would actually look more like this...

```
<html xmlns="http://www.w3.org/1999/xhtml" <?php print backdrop_attributes($html_attributes); ?>>
```