Replace pairing over Wi-Fi hotspot with Bluetooth Low Energy

[davidknezic]

The currently implemented Wi-Fi hotspot-based pairing mechanism is extremely unreliable. As it turns out, this is due to several reasons, one of the most important ones being that the Raspberry Pi Wi-Fi chip only supported the hotspot and normal Wi-Fi connection to be on the same channel. It also doesn't support Channel Switch Announcments (CSA). This leads to a very clumsy UX, also because iOS needs to prompt the user every time a Wi-Fi connection is made from an app on their behalf.

This is why this PR introduces BLE-based pairing.

It introduces a BLE service called Candy:

CA000000-75dd-4a0e-b688-66b7df342cc6

That service provides the following characteristics:

candyServiceUuid
networkAvailabilityStatus
ipAddress
wifiScanList
wifiSsidString
wifiPskString
wifiConnectSignal

Those characteristics allow an app to discover Candy Dispensers nearby, list the Wi-Fi networks they can reach, to instruct them to connect to a Wi-Fi and to get status information.

The GATT service can be tested though the LightBlue app.