"Participating" notifications feature

[jgoz]

I'm looking at adding an option to get "participating notifications" only (i.e., setting participating=true in the API call).

It looks like that would involve adding a new (nullable) participating column to the records table. Is that something you would be open to? I'm thinking it would become a second constructor parameter on Client and get added to the request if it's truthy.

The corresponding issue in the app repository (CodeHubApp/CodeHub#375) would be handled by adding a new "Participating" toggle option. It would only be enabled if push notifications are enabled, and toggling it on/off would have to do a register/unregister cycle to propagate the option value to the push server.