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

PR for https://github.com/backdrop/backdrop-issues/issues/200
There should not be any quotes around the call to backdrop_attributes(), otherwise you end up with invalid HTML. I think this should be:
```
<html xmlns="http://www.w3.org/1999/xhtml"<?php print backdrop_attributes($html_attributes); ?>>
```

With no space before the opening PHP tag and no quotes.