Add input -> RC, and RC -> function mapping UI

[steveatinfincia]

We will need the ability to map USB HID "channels" to RC channels, to allow users to configure which inputs will be sent over the air and passed on to the flight controller.

It's certainly easier to hardcode a specific number of axis/switches/buttons but that won't work for everyone, and we don't want to assign 2 functions to a specific channel either. You wouldn't want channel 7 to be configured for camera control, and then also send it to the flight controller and have someone forget they configured it as a RTL function. In those cases we would probably send the neutral RC value to the flight controller for that particular channel.

We will also need to map RC channels to specific functions, like bandwidth switching and camera control. Those two are both currently reusing the same control system over-the-air, but they will be separate soon and there will be other things people may want to map that way as well like camera controls mapped to a button or dial, or GPIO control mapped to a switch.

This requires some coordination on the ground side, probably in the microservice code.

Ideally the UI would just be a list of dropdown menus on the left and right side of the screen with input on the left and functions on the right, and we may end up needing to choose which ones go over the air based on what they're configured to do, otherwise we'll end up with 2 entirely separate configuration panels people have to deal with. That will require more consideration.