feat: see people available within selected range in group availability

[RaymondWHZ]

We can allow the user to select some timeslots in group availability to see who is available across all the selected timeslots. For example, one can choose to see all people available from 9:00 am to 12:00 pm. We can implement this by taking back the original function of the scheduler component instead of directly disabling selection.
Completing this feature will also facilitate responsive design: mobile users can thus select some slots and click some "unfold" button to see all available people, eliminating hovering logic which is not easy to perform on mobile.