Support collective stats by joining a call group

[nickoneill]

We've had a couple requests for folks to count calls as a group, not just individually. We'll accomplish this by being able to set an arbitrary string as a group identifier in the settings screen which will be passed as a param with call results.

Tracking some details for various platform support here:
https://github.com/orgs/5calls/projects/3