Fix Handling of "Only Show Admins" Filter

[aaronskiba]

Fixes # .

Changes proposed in this PR:

• Updated the index action within Paginable UsersController to include filter_admin in the query_params for paginable_renderise.
• This addresses the issue where filter_admin was sometimes nil within the controller, even when it was set to '1' in the browser (https://github.com/portagenetwork/roadmap/issues/941).