Rework WiFi code

[gorbit99]

This PR reworks the WiFi code in the following ways:

• WiFiNetwork and WiFiProvisioning are now singleton classes, instead of namespaces with free-floating functions in them
• The code for the wifi connection logic is now pulled into separate functions for clarity
• The mcu only tries switching to the G phy mode if the connection on the N phy mode timed out, not when the credentials were incorrect
• The reason for the failure is now reported correctly

[gorbit99]

Since the ESP8266 and the ESP32 code didn't agree on the status codes, I remapped them to the following error codes instead:
0 - Timeout (likely DHCP failure)
1 - SSID not found
2 - Wrong password
3 - Unknown error (didn't manage to trigger this)

The new format for the message that the connection failed is
Can't connect from any credentials, error: <error code>, reason: <error message>.