Twenty Seventeen: Ensure the use of proper image size for custom header image

The `sizes` attribute was calculated incorrectly, resulting in the browser downloading the wrong resource. This filters the attribute to correct that.

Before the filter, the `img` contained:
`sizes="(max-width: 767px) 89vw, (max-width: 1000px) 54vw, (max-width: 1071px) 543px, 580px"`

And after:
`sizes="100vw"`

Props joemcgill, transl8or.

Fixes #38847.

Built from https://develop.svn.wordpress.org/trunk@39291


git-svn-id: https://core.svn.wordpress.org/trunk@39231 1a063a9b-81f0-0310-95a4-ce76da25c4cd