Twenty Seventeen: Adds `background-attachment: fixed;` to devices that should support it

iOS  disables this feature under the hood, but it also distorts the images – unlike other mobile devices that don't support it. So this adds a  check for both `background-attachment: fixed` support or whether it’s an iOS device - passing it adds the class `background-fixed` which is used to add the proper styles.

It also lowers the media query so the parallax-like style is present on a wider range of screens since this bug can be better targeted and avoided. In this way, screens that aren't the offending devices aren't punished merely based on screen size.

Props joemcgill, laurelfulford, helen.

Fixes #38395.

Built from https://develop.svn.wordpress.org/trunk@39297


git-svn-id: https://core.svn.wordpress.org/trunk@39237 1a063a9b-81f0-0310-95a4-ce76da25c4cd