Update/newsletter category settings

[allilevine]

Fixes https://github.com/Automattic/loop/issues/281

Proposed changes:

• Change the newsletter categories toggle to save only when "Save Settings" is clicked.
• Add "Which categories would you like to allow people to subscribe to? Select all that apply:" above the newsletter category selection.
• Show an error message when newsletter categories are enabled, but none are selected: "Please select at least one category when newsletter categories are enabled."
• No go: adding testing for newsletter settings. I attempted to but mocking all the required data got complicated fast. There are tests for settings in general.
• Note there's only one Save Settings button you can use to save the categories settings.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1734114241935889/1733936868.332319-slack-C083ZPVVDTK

Does this pull request change what data or activity we track or use?

N/A

Testing instructions:

• On a Jetpack or Atomic site, go to /wp-admin/admin.php?page=jetpack#/newsletter and scroll to Newsletter categories.
• Enable categories.
• Without selecting any categories, click "Save settings."
• You should see an error, and settings won't save.
• Select one or more categories.
• Settings should save successfully.
• Disable categories.
• Settings should save successfully.
• Start with a site that currently has categories enabled but none selected. Check that the page still loads and you can select categories.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the update/newsletter-category-settings branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack update/newsletter-category-settings
  

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[allilevine]

Fixed a typo here.