Implement Reactive Embed Updates for Raid/Headcount Instances

[dmansdman]

Current procedure is to use 4-second intervals to update all panels. This can definitely lead to the bot feeling slow (waiting up to 4 seconds to see your reaction take place), and might be a cause for slow performance.

I propose trialing a reactive system where each reaction leads to the embed being updated immediately. We can retain periodic interval updates at a much greater interval, or remove them entirely.