Responsive images: add compatibility for versions < 2.7 when the full image...

Responsive images: add compatibility for versions < 2.7 when the full image path was stored in the metadata. Introduces `_wp_get_attachment_relative_path()` and uses it in `wp_get_attachment_url()`.

Props dd32, SergeyBiryukov.
Fixes #35106 for trunk.
Built from https://develop.svn.wordpress.org/trunk@36120


git-svn-id: https://core.svn.wordpress.org/trunk@36086 1a063a9b-81f0-0310-95a4-ce76da25c4cd