Use random mqtt ID

This commit changes a few things related to MQTT, but the most
important is that it removes the set MQTT Client ID.  This mitigates
an issue wherein the Gizmo could disconnect and reconnect and the
broker would expect it to inherit the previous session, but the MQTT
library in the current software doesn't know how to do that.  This
results in a noticable pause while the subscriptions from the old
instance age out before new data is delivered.

config.h: Changes I2C pins to match hardware.

.ci.sh: Build for multiple boards

Re-request config if not received.

.github/workflows: Add release workflow