Accessibility: Themes: Only unlink custom logo on the home page if the theme...

Accessibility: Themes: Only unlink custom logo on the home page if the theme declares support for that.

To accommodate for the change, the custom-logo theme feature now accepts the unlink-homepage-logo parameter.

If and when the logo is linked on the home page, the link has the aria-current attribute for better accessibility.

Follow-up to [48039], [48283], see [48749].

This brings the changes to the 5.5 branch.

Props sabernhardt, audrasjb, johnstonphilip, demetris, SergeyBiryukov.
Fixes #37011.

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git-svn-id: https://core.svn.wordpress.org/branches/5.5@48517 1a063a9b-81f0-0310-95a4-ce76da25c4cd