Disable lightbox toggle for image linking to attachment pages

[swissspidy]

Feature description

This is a follow-up to #3450. As proposed in https://github.com/ampproject/amp-wp/pull/3460#pullrequestreview-304677157, "the lightbox toggle should probably be prevented from even being made available when the image block has a link to the attachment page. Otherwise, the lightbox toggle has no effect when an attachment page is linked."

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

@jamesozzie pointed out that images in AMP pages now automatically get lightboxing. See https://github.com/ampproject/amphtml/blob/master/spec/auto-lightbox.md

Do we even need the lightbox toggle anymore?

[amedina]

Do we even need the lightbox toggle anymore?

Closing (as per comment above) and will reopen if needed.

[westonruter]

Let's leave open because we should remove the toggle if indeed it is unnecessary.

[westonruter]

Closely related issue: #4591

[westonruter]

Attachment pages are disabled in 6.4. Also, in 6.4 lightbox images are now supported in core. So in https://github.com/ampproject/amp-wp/issues/7676 we're going to eliminate the AMP toggle when on 6.4. Otherwise, it doesn't seem necessary to anything further with this issue.