Add a filter to allow social services edits

[imath]

What?

This PR introduces the block_core_social_link_services filter so that it's possible to add a new social links to the corresponding block.

Why?

• There will always be new social services the block_core_social_link_services() function might not include.
• Used inside a corporate Intranet, it would be the only way to include corporate social inks.
• It can also be used to restrict/disable unwanted social links by administrators.

How?

This PR introduces the block_core_social_link_services filter inside the block_core_social_link_services() function.

[talldan]

Thanks for the contribution.

I remember some discussion on this before, and there's an issue that the services and associated icons are specified in two separate places—in the JS code for the block and in the PHP code (which this PR touches).

I think filters would need to exist for both, though for the frontend code users can already specify variations, which could mean a filter isn't needed.

It'd be good to see some docs for this so that devs know how to implement the full experience.

[github-actions]

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• Labels found: [Feature] Extensibility, [Status] Stale, [Block] Social.

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