Switch to small feed topic format

[brentru]

We may want to switch from /feeds/ and /groups/ to the small topic feed format to conserve space.

{your Adafruit IO username}/g/{group key}
{your Adafruit IO username}/g/{group key}/csv
{your Adafruit IO username}/g/{group key}/json
{your Adafruit IO username}/f/{feed key}
{your Adafruit IO username}/f/{feed key}/csv
{your Adafruit IO username}/f/{feed key}/json

https://io.adafruit.com/api/docs/mqtt.html#small-topic-format