Re-implement beeper feedback

[Knifa]

The beeper doesn't beep any more when you press buttons or do things.

I think the behaviour before was:

• Beep on any button press.
• Beep on "system" activity like saving or low voltage.
• Beep on low RSSI.

We can make use of the button hooks to beep on button press, but perhaps it should only beep when it actually does something?

[sheaivey]

On the LaForge we beep when rx modules switch and button presses.

[lovejoy777]

beep on rx switching, low rssi and voltage alarms would be good. personally not bothered about button press (although would be nice) as my diy hardware has the 3 tactile switches which click quite loud when pressed. Just my 2 cents 😉. keep up the great work guy's.