Better guide users when deleting other users in single- and multisites

[derpaschi]

This pull request refactors and improves the user deletion workflow in multisite and single site WordPress admin. The main focus is on making the "Delete Users" form more accessible, robust, and user-friendly by ensuring that deletion/reassignment options are explicitly selected for each user, and by modernizing the associated JavaScript and PHP logic.

Key improvements and bug fixes:

1. User Deletion Form Usability and Accessibility

• Refactored the "Delete Users" form so that each user now has their own set of radio buttons for deletion or content reassignment, instead of a single global option. This ensures that admins make an explicit choice for each user, reducing the risk of accidental data loss. [1] [2] [3]
• The form now disables the submit button until all required options are selected, improving accessibility and preventing incomplete submissions.
• Added clearer labeling and structure to the form, including using fieldsets and legends, and updating the page heading to "Delete Users" for clarity.

2. JavaScript Modernization

• Removed legacy inline JavaScript (delete_users_add_js) and replaced it with a modern, unobtrusive script in common.js that manages form validation and submit button state. [1] [2] [3]

3. Backend Logic and Data Handling

• Updated backend handling to process per-user deletion/reassignment options using associative arrays, ensuring each user's content is handled as intended. [1] [2]
• Improved detection of whether users have associated content, both in single and multisite, to determine which options to present in the UI. [1] [2]

4. Code Cleanup and Consistency

• Removed outdated code and improved variable naming for consistency and clarity throughout the affected files. [1] [2] [3] [4] [5] [6]

5. Multisite-Specific Enhancements

• In multisite, the form now correctly excludes the users being deleted from the reassignment dropdowns and handles cases where users have no content on a given site.

These changes collectively make user deletion safer, more intuitive, and more maintainable for WordPress administrators.

Screenshots Multisite

Screenshot Singlesite

Test steps

• Create up to 3 users (single or multisite)

• Add posts and assign posts to the created users as author

• Delete one of the users and select the user the content should be assigned to
  or

• Delete multiple users and select another user for attribution

• Deleting users should not be possible if not all users that will be deleted have a selection for their content attribution

• You should not be able to select a user for content attribution that will be deleted

• No default selection for content attribution, so you have to actively decide what should happen.

• When deleting multiple users in single-site installations, you can now select what should happen with their contents individually

Trac ticket: https://core.trac.wordpress.org/ticket/56914

Contributors
@ocean90 @krokodok @velthy @KarinChristen

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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[github-actions]

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[krokodok]

These changes look very good!