Add social_notifications_subscribe option in WP Admin

[htdat]

Fixes #709

When /wp-admin/options-discussion.php is fully loaded, the option looks like this:

Changes proposed in this Pull Request:

Basically, I followed the same approach for another similar option Someone likes one of my posts in "Likes" module https://github.com/Automattic/jetpack/search?q=social_notifications_like&type=Code

• Handle social_notifications_subscribe option when "Subscriptions" module is activated/deactivated.
• Add Someone follows my blog in WP Admin -> Settings -> Discussions.
• Use jQuery to align up this option in the Email me whenever section.
• Validate its value to only on/off.
• Add this option to \Jetpack_Core_Json_Api_Endpoints class.

I do not add tests as social_notifications_subscribe is covered already. Can a dev help to double check this?

Is this a new feature or does it add/remove features to an existing part of Jetpack?

• This is more an enhancement. See p9o2xV-3Q-p2 and Send email to site owner when the site gets a new email subscriber #709 (comment)

Testing instructions:

• Disable Subscriptions module => the option should not display in "Discussions" settings
• Enable Subscriptions => the checkbox of this option should not be selected by default.
• Enable this option, try to subscribe with a new email => the Jetpack connection owner gets an email notification about the new email.

Proposed changelog entry for your changes:

• Add Email me whenever: Someone follows my blog option to wp-admin settings.

[jetpackbot]

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[zinigor]

This change works well for me, but I have several comments. I don't really like how we're copying the same code that's used in the Likes module, but I guess that's the quickest way to fix the issue that we're aiming at.
Thanks for working on this!

[htdat]

@zinigor Thanks for your thorough review. I've added a new commit to address your suggestions.

I don't really like how we're copying the same code that's used in the Likes module, but I guess that's the quickest way to fix the issue that we're aiming at.

Well, me too, I do not like this approach much. But since we have moved new options to Calypso or Jetpack setting pages, it's not efficient to refactor related code.

[zinigor]

Thanks for fixing everything! I have added the changes that linter required, please don't skip checks when you commit.