Add pagination to /admin/users/list API

Prior to this, only the first 100 active/new/etc. users were available
via the `/admin/users/list` API.  This change adds support for a
`page=#` querystring parameter so that *all* of the users can be
retrieved.  Requests for pages past the last user result in an
empty-list response; requests for negative pages (or zero) just return
the first page.

Added tests to cover pagination.

lib/admin_user_index_query.rb

@@ -25,7 +25,11 @@ class AdminUserIndexQuery
   }
 
   def find_users(limit=100)
-    find_users_query.limit(limit)
+    page = params[:page].to_i - 1
+    if page < 0
+        page = 0
+    end
+    find_users_query.limit(limit).offset(page * limit)
   end
 
   def count_users