UI for switching calendar view (day/week/month)

[arhanjain]

Added a UI to the HeaderControl component to switch between calendar views. Also added HeaderControl to Month view. Day button is currently commented out because Day view component doesn't exist yet.

I'm getting familiar with the codebase with these small contributions so that I can make more meaningful contributions in the future. Let me know if there are any suggestions or improvements I can make with my code.