Jetpack Settings: Add Theme Enhancements card under Writing Settings for Jetpack sites

[tyxla]

Introduction

This PR is part of #9171. It introduces a Theme Enhancements card under Writing Settings. This card will be used to manage the settings of the Infinite Scroll and Mobile Theme Jetpack modules.

Preview

Both modules activated, some settings are disabled

Initial loading

Loading while saving

Disabled modules (settings are hidden)

Testing

• Checkout this branch.
• Select one of your Jetpack sites with Jetpack 4.5 or newer.
• Go to /settings/writing/$site, where $site is the slug of your Jetpack site.
• Verify the Theme Enhancements card is displayed properly.
• Play with the main toggles that enable the modules, and verify they save correctly in your Jetpack site (note that notices are implemented separately in Jetpack Settings: Add notices for (de)activating Jetpack modules #10583).
• Verify that the settings of a module are displayed only when the module is active.
• Play around with the settings of each module and click "Save Settings". Verify they save properly in your Jetpack site.
• Verify the links of the info popovers are working as expected and lead to the right place.
• Test with a Jetpack site that has Jetpack 4.4.2 or older, and verify the card is not shown.
• Test with a WordPress.com site and verify that the card is not shown.

[matticbot]

Test live: https://calypso.live/?branch=add/jetpack-setings-theme-enhancements-card

[beaulebens]

Can we get a screenshot/mock of what we should be seeing for this?

[beaulebens]

You might not have done this one yet, but this appears to be the wrong helper/information link for this setting.

[tyxla]

Can we get a screenshot/mock of what we should be seeing for this?

Sure thing, I've updated the PR description to contain a screenshot. I'll also update with some testing instructions and additional information once the PR is ready to review.

You might not have done this one yet, but this appears to be the wrong helper/information link for this setting.

Yeah, this was in progress, but I've now updated it, too. Could have missed it, so good catch! 👍

[tyxla]

This PR works and saves data properly; however it will be best if we adapt it to using the bulk settings endpoint; this is being implemented in #10123.

[tyxla]

I've adapted this PR to use bulk settings from #10123, and done a ton of other improvements. I've also updated the PR description with new screenshots, and a TODO list with the remaining stuff.

[roccotripaldi]

Code looks good. Everything works smoothly, and as described, even when switching between sites.