Add import source users pagination info to GroupMembersHelper

What does this MR do and why?

This MR adds placeholder user counts to GroupMembersHelper in order to use this information to show 'Placeholder' tabs on the group Members page. It adds the following info:

{
  placeholder: {
     total_items: placeholder_users.count, # total number of placeholder users for this namespace
     awaiting_reassignment_items: placeholder_users.pending_assignment.count, # number of placeholder users waiting for reassignment
     reassigned_items: placeholder_users.completed.count # number of placeholder users that were reassigned to real users 
  }
}

Mentions #468437 (closed)

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