Allow site administrators to change the default editor for other users.

[jakeparis]

Currently, site administrators and superadmins can't change the default editor for users on their site. This PR adds that functionality.

[ironprogrammer]

Thank you for contributing to Classic Editor, @jakeparis!

This is a useful update that can make administrators' (and their users'!) lives easier 👍🏻 I've rebased the PR along with a minor tweak to the user ID variable name.

[ironprogrammer]

Test Report

Steps to Test

1. As an admin user, in Settings > Writing, enable the "Allow users to switch editors" setting.
   
2. From the Users screen, edit another user (not your own).
3. Change the "Default Editor" choice for the user and click Update User to save their profile.
   
4. Using a plugin such as User Switching or a separate browser, log in as the updated user and go to Posts > Add New Post.
5. Confirm that the default editor type matches what was set by the admin user.

Expected Results

• ✅ Admin users can set the default editor for other users.
• ✅ Users' default editor option matches that set by an admin.

Environment

• Hardware: MacBook Pro Apple M1 Pro
• OS: macOS 14.5
• Browser: Safari 17.5
• Server: nginx/1.27.0
• PHP: 8.2.20
• MySQL: 8.0.27
• WordPress: 6.6-beta3-58440-src
• Active Plugins:
  • classic-editor v1.6.3
  • user-switching v1.7.3

Actual Results

• ✅ Admin users can set the default editor for other users.
• ✅ Users' default editor option matches that set by an admin.
• ✅ Also verified that users can reset their own default editor, and that selecting an editor type per-post works as expected.