Fixed shifts filter to show partially occupied shifts as free and not occupied

As suggested by @pjdeltour in #829 (Shifts filter - not showing all filtered shifts)

includes/controller/users_controller.php

@@ -408,14 +408,14 @@ function shiftCalendarRendererByShiftFilter(ShiftsFilter $shiftsFilter)
 
         if (
             in_array(ShiftsFilter::FILLED_FREE, $shiftsFilter->getFilled())
-            && $taken < $needed_angels_count
+            && $needed_angels_count > 0
         ) {
             $filtered_shifts[] = $shift;
         }
 
         if (
             in_array(ShiftsFilter::FILLED_FILLED, $shiftsFilter->getFilled())
-            && $taken >= $needed_angels_count
+            && $needed_angels_count == 0
         ) {
             $filtered_shifts[] = $shift;
         }