Currently, the main connection flow starting from /wp-admin/admin.php?page=jetpack#/ has the connect-in-place flow, but the buttons starting from the WP-admin dashboard (/wp-admin/index.php) and plugins screen (/wp-admin/plugins.php) still work the old way.

This PR updates these buttons so they respect the constant and A/B test, and based on them, redirect to either the original connection flow or the connect-in-place flow.

Changes proposed in this Pull Request:

• Update dashboard and plugins screens connection buttons:
  • Respect the JETPACK_SHOULD_USE_CONNECTION_IFRAME constant.
  • Respect the jetpack_connect_in_place_v2 A/B test.
  • If connect-in-place flow is enabled through the constant or the A/B test, redirect to it and start the flow.
  • If connect-in-place flow is not enabled, the buttons would redirect to the WP.com connection flow, like they used to do before.

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

• Part of p1HpG7-7nj-p2

Testing instructions:

• Checkout this branch.
• Build Jetpack npm build.
• Follow the instructions below in the following order. I've separated them in sections for clarity.
• Test the connect in place flow from the Jetpack dashboard
  • Disconnect Jetpack if it was connected.
  • Add this to your wp-config.php: define( 'JETPACK_SHOULD_USE_CONNECTION_IFRAME', true );
  • Go to /wp-admin/admin.php?page=jetpack#/
  • Click the "Set up Jetpack" button.
  • Verify it works the same way as it does on master.
  • Verify you're still able to connect like before.
  • Disconnect Jetpack.
• Test the connect in place flow from the WP admin dashboard
  • Go to /wp-admin/index.php.
  • Click the "Set up Jetpack" button.
  • Verify it redirects to /wp-admin/admin.php?page=jetpack#/setup
  • Verify you see the loading screen, and after a little, you see either the "Approve" button, or the "Connect with WordPress.com" button.
  • Verify you're still able to connect like before.
  • Disconnect Jetpack.
• Test the connect in place flow from the plugins screen
  • Go to /wp-admin/plugins.php.
  • Click the "Set up Jetpack" button.
  • Verify it redirects to /wp-admin/admin.php?page=jetpack#/setup
  • Verify you see the loading screen, and after a little, you see either the "Approve" button, or the "Connect with WordPress.com" button.
  • Verify you're still able to connect like before.
  • Disconnect Jetpack.
• Test the original connection flow from the Jetpack dashboard
  • Remove this from your wp-config.php: define( 'JETPACK_SHOULD_USE_CONNECTION_IFRAME', true );
  • Go to /wp-admin/admin.php?page=jetpack#/
  • Open your Network requests tab.
  • Click the "Set up Jetpack" button.
  • Verify that this performs a request to the A/B test endpoint.
  • Verify you're redirected to the WP.com Jetpack Connect authorization flow.
  • Verify you're still able to connect like before.
  • Disconnect Jetpack.
• Test the original connection flow from the WP admin dashboard
  • Go to /wp-admin/index.php.
  • Open your Network requests tab.
  • Click the "Set up Jetpack" button.
  • Verify that this performs a request to the A/B test endpoint.
  • Verify you're redirected to the WP.com Jetpack Connect authorization flow.
  • Verify you're still able to connect like before.
  • Disconnect Jetpack.
• Test the original connection flow from the plugins screen
  • Go to /wp-admin/plugins.php.
  • Open your Network requests tab.
  • Click the "Set up Jetpack" button.
  • Verify that this performs a request to the A/B test endpoint.
  • Verify you're redirected to the WP.com Jetpack Connect authorization flow.
  • Verify you're still able to connect like before.
• If you got to this point, I wish to thank you for the thorough testing ❤️💪

Proposed changelog entry for your changes:

• Connection: Redirect plugins and dashboard connect buttons to the main flow