Accessibility: Themes: Don't link to the home page in `get_custom_logo()` when...

Accessibility: Themes: Don't link to the home page in `get_custom_logo()` when it's displayed on the home page.

Props Soean, audrasjb, sabernhardt, FlorianBrinkmann, rianrietveld, afercia, joedolson, samful, knutsp, SergeyBiryukov.
See #37011.
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