Improve efficiency of user queries performed by the AdminUI
To make the search box on AdminUI->Organization->Users work, a not trick implementation was implemented to have something working at all.
That implementation essentially gets all users from the UserDirectoryService and then manually performs filtering, sorting, pagination and searching.
This could become problematic in case that both a large number of users are available at the UserDirectoryService and a large number of users are accessing the page AdminUI->Organization->Users.
A better solution would be the use of a search index for the users, so that all the filtering, sorting, pagination and searching could be done more efficiently.