User fields: Some remaining uses of deprecated functions

The only substantive code change is a change to the get_ws_authorised_users function, only used by service_users.php, here is how to test that page:

1. Add a custom user profile field to your test Moodle (Users > Accounts > User profile fields). The field should be a text type and with max length e.g. 100 characters (otherwise it doesn't appear on the list for the next step).
2. Add this field in the 'show user identity' option (Users > Permissions > User policies). You can leave whatever other fields you like ticked, e.g. 'email' from default.
3. For a test user account, edit their profile and set a value for the new user profile field.
4. Go to the external services page (Plugins > Web services > External services) and click Add
   • Set the name and short name to any suitable values you like, but make sure to tick the 'Authorised users only' box.
   • When you get to the 'Add functions to the service...' page, don't add functions. Instead, go back to the 'External services' page from the breadcrumbs.
5. Click on the 'Authorised users' link next to your new service.
6. In the list of unauthorised users, search for and select the user where you set up the account, and click 'Add'.
   • NOTE: The custom profile field ideally ought to display in the selector and be searchable, but that is an existing TODO elsewhere in the code not implemented here - so in the selector, you won't see the custom field, and that's OK.
7. Look below the selector.
   • You should see the list of authenticated users including the user you just added.
   • Below the name of the user it should show all the user identity fields including the custom field