Custom Posts: Support "Access" and "Email to Subscribers" options

[crazytonyli]

Note

I recommend reviewing by commits. The first commit is pure code format changes and can be ignored.

Description

Fixes https://linear.app/a8c/issue/CMM-2069.

Custom Posts settings now support the same Jetpack newsletter options as regular Posts:

1. The Access section shows the Jetpack newsletter access level.
2. The Publish sheet shows "Email to Subscribers".
3. The selected access level and email setting are read from and written back to REST meta.

This PR also fixes a bug where these two options remain visible even when the "Subscriptions" Jetpack module is not activated.

Testing instructions

"Subscriptions" Jetpack module must be activated, which is the case for all simple sites. For jetpack sites, you'll need to go to the Newsletter -> turn on "Let visitors subscribe to this site and receive emails when you publish a post".

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Configuration	Release-Alpha
	Build Number	32271
	Version	PR #25574
	Bundle ID	org.wordpress.alpha
	Commit	9805872
	Installation URL	65a5ge40dh0ho

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Configuration	Release-Alpha
	Build Number	32271
	Version	PR #25574
	Bundle ID	com.jetpack.alpha
	Commit	9805872
	Installation URL	1cf44h2tlqh2g

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[wpmobilebot]

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