Connection: Move out the JSON API auth hooking

[tyxla]

In #12699 we forgot to port the JSON API auth functionality to the connection package 😱 You can see that there's a reference to $this->login_form_json_api_authorization in the connection manager, but that method doesn't exist 😕

This PR puts it back to the Jetpack class where it belongs, as the method we're using still lives there.

This is a better alternative than the approach initially suggested in #13224.

Changes proposed in this Pull Request:

• Connection: Move out the JSON API auth hooking.

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

• This is a continuation of this effort: p1HpG7-7lI-p2

Testing instructions:

• https://developer.wordpress.com/docs/oauth2/
• Can anyone figure out better ways to test this?

Proposed changelog entry for your changes:

• Connection: Move out the JSON API auth hooking bit

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: September 3, 2019.
Scheduled code freeze: August 27, 2019

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[mdawaffe]

💥