Sharing Service: Make sure we only check for legacy sharing buttons implementation on non-block themes

[oskosk]

Makes it so we only check for the need to load legacy sharing assets if we're on an old theme

Saves a call to get_option( 'sharing-services' ) for modern themes in the frontend.

Proposed changes:

• Adds a function sharing_should_load_legacy_sharing_buttons() that checks if the theme is a modern theme
• Updates sharing_add_header() to check with sharing_should_load_legacy_sharing_buttons() before checking if it should load the sharing assets in the frontend

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pdWQjU-Ub-p2

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Testing instructions:

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[github-actions]

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[jeherve]

Part of me wonders if we shouldn't take that opportunity to be way more aggressive, and shortcircuit all of the sharing feature when you use a block-based theme. Ideally you really shouldn't be using the legacy buttons in a block-based theme, you should be using the sharing block instead.

We already recommend that people use the sharing block instead:

After merging this PR, maybe we could spin up a small project to more strongly redirect folks to the sharing block?

[oskosk]

After merging this PR, maybe we could spin up a small project to more strongly redirect folks to the sharing block?

Tracking here
Sharing: Disable legacy sharing frontend mechanism for block based themes #39358