REST API: Add the missing 'site_icon_url' to the index

[Mamaduka]

Backports changes from WordPress/gutenberg#42957.

The site_icon_url index was supposed to ship with WP 5.6 but was never backported into the core - WordPress/gutenberg#22952.

I also created an alternative PR to use the existing site_icon index, but later @TimothyBJacobs pointed out that site_icon_url is preferable since it can be filtered - WordPress/gutenberg#43514 (comment).

Gutenberg tracking issue: WordPress/gutenberg#43440
Trac ticket: https://core.trac.wordpress.org/ticket/56467

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[ockham]

Thank you very much, @Mamaduka! 🙌

Some unit tests are failing; apparently, tests/qunit/fixtures/wp-api-generated.js needs updating:

Furthermore, I've glanced over WordPress/gutenberg#42957 and WordPress/gutenberg#43514. I'm not entirely sure about the conclusion -- it seems like we won't need site_icon_url once we merge WordPress/gutenberg#43514; but we still need to backport it for backwards compatibility. Is that correct? 😊

[Mamaduka]

I am running a PHP test suite to regenerate fixtures, and I will push the fix shortly.

Furthermore, I've glanced over WordPress/gutenberg#42957 and WordPress/gutenberg#43514. I'm not entirely sure about the conclusion -- it seems like we won't need site_icon_url once we merge WordPress/gutenberg#43514; but we still need to backport it for backwards compatibility. Is that correct? 😊

We decided to use site_icon_url index, so that it can be filtered via get_site_icon_url. I'll close #43514 after this is backported.

[ockham]

LGTM! Thanks @Mamaduka 🚀

[TimothyBJacobs]

May make sense to move this logic into add_site_icon_to_index.

[Mamaduka]

Thanks for the suggestion, @TimothyBJacobs. I've updated the PR.

[ockham]

(Re-approving the updated PR.)

[SergeyBiryukov]

Thanks for the PR! Merged in r54083.