OSIS-3397 update css to center spinner
OSIS-3397 update css to center spinner

uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/OSIS-Louvain,uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis,uclouvain/osis