Upgrade Nudge: Fix redirect_to for Jetpack wp-admin

[ockham]

Required for #13081, and implementing musings found at #13081 (comment). Related: Automattic/wp-calypso#35027.

Changes proposed in this Pull Request:

Change the post-checkout redirect target to the wp-admin block editor for Jetpack sites, both in the editor (JS), and the frontend (PHP).

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

Modifies the existing UpgradeNudge.

Testing instructions:

• Switch to this branch locally, build Jetpack (yarn build), and start Docker (yarn docker:up)
• Now add the following line to any file in docker/mu-plugins, e.g. a newly created 0-blocks-upgrade.php (needs to start with <?php): define ( 'JETPACK_SHOW_BLOCK_UPGRADE_NUDGE', true );
• Make sure you're connected to WP.com, and on a free plan
• Start a new post, and insert the 'Simple Payments' block
• Select the block, and enter some information into its text fields.
• Click the nudge's "Upgrade" CTA button
• Pay for the premium plan
• Verify that you're redirected back to where you came from (the wp-admin block editor) (without any intermediate redirects on WP.com)

Verify that on WP.com, the redirect target is still the iframed Gutenberg editor (well, if you have enabled it, see PCYsg-hNB-p2):

• Apply the WP.com counterpart patch (D31062-code) to your sandbox
• Make sure your sandbox is on a free plan, and that you have enabled it to use Gutenberg
• Open the block editor for your sandbox site from within Calypso
• Insert the 'Simple Payments' block
• Click the nudge's "Upgrade" CTA button
• Pay for the premium plan
• Verify that you're redirected back to where you came from (the iframed Gutenberg editor in Calypso)

Proposed changelog entry for your changes:

N/A (The UpgradeNudge hasn't been enabled in any previous JP version, so no need to add a ChangeLog entry.)

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello ockham! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D31062-code before merging this PR. Thank you!

[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: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against 8a768f9

[matticbot]

ockham, Your synced wpcom patch D31062-code has been updated.

[matticbot]

ockham, Your synced wpcom patch D31062-code has been updated.

[matticbot]

ockham, Your synced wpcom patch D31062-code has been updated.

[ockham]

Unfortunately, we don't seem to be exposing the site's protocol through any utilties available to JS (neither from Gutenberg/core nor Jetpack), so I'm falling back to window.location.protocol.

Devil's advocate: Why even bother constructing the URL from 'reliable' data and not just window.location.href for the entire redirect_to URL I'm building?

[jeherve]

This seems to test well on Jetpack (I tested on both HTTP and HTTPS sites). I have not tested on WordPress.com though. I'll let someone else review that!