An imelementation of Agora Video Call

• Based on official example code.
• More streamlined user flow: fill in necessary info first, then join channel
• Check required piece of info before joining
  • if camera or microphone not present, refraine user from joining.
• Action buttons only appear/clickable when they are useable
• Unpublishing stream automatically resets audio to "not muted."
• State management with useReducer
  • Use enum to manage state when possible (possible improvement: use finite state machine to further simplify, avoiding bug down the road).
• Use async, await to improve readability.