Switch to AsyncWebserver - Laying the groundwork for SSL

[Eroc33]

I'm somewhat assuming that the most likely way to get https on the esp32 is AsyncWebserver (Whenever they actually implement SSL for the esp32 port of ESPAsyncTCP).

This doesn't seem to impact performance particularly, and even though it doesn't give us SSL at this time it gives a few neat features to play with in the future, such as web-socket server and client which might be useful for streaming data either to the web interface, or data logging services.

[layerzerolabs]

Looks super neat! And we're most of the way to closing #14 - we already have a favicon.ico file! https://github.com/hamishcunningham/fishy-wifi/blob/master/ardesp/waterelf/data/favicon.ico