One URL for all Adafruit boards

[don]

It would be nice to have one URL for all Adafruit boards for easier IDE configuration.

https://apt.adafruit.com/proxy_files/package_adafruit_index.json works for now, but isn't on the list of 3rd party boards.

[toddtreece]

fixed by 9102d17 and added the new URL to the list of 3rd party boards.