Display all users (existing and removed) in the Users Group listing

The main purpose of this ticket is to manage LDAP users added to local groups when some LDAP users have been removed/disabled from  LDAP.

The easiest way to reproduce is to work with a SQL database and update the table "user2group" to add a phantom user, which will help to understand the requirement.

1. First step: add the phantom user to the table "user2group"

   INSERT INTO public.user2group(
   	"groupId", "userId")
   	VALUES ('members', 'jack');
   

2. log in to Nuxeo as Administrator
3. navigate to Administration > Users & Groups
4. search the user "Administrator"
5. click on the user "Administrator" in the results
6. on the User summary, click on the button "Add to group"
7. search for "members" and select the group => Administrator is now added to the "members" group
8. go back to Administration > Users & Groups
9. search the group "members"
10. click on the group "members" in the results
11. observe that only "Administrator" is listed in the members of this group

Current behavior:
user "jack" is not listed on the Group summary page, and it is not possible to use the X icon to remove it from the group

Expected behavior:
user "jack" should be listed on the Group summary page with an icon to indicate that the user cannot be found in the User directory, and there is the X icon on the line for "jack" to remove it from the group

Note that the API call which reads the properties of the "members" group returns "jack" among the members, so it should be possible to use this information to display the missing user.