Improve internal model of ringing, expose ringing intent to call UI

I found our code's internal model of ringing a little overgrown (it had superfluous states like 'unknown') and difficult to extend with metadata or callbacks relating to ring attempts. By modeling ringing instead as a stream of ring attempts, where each attempt has an intent, a recipient, and an eventual outcome (accept/decline/timeout), I find it more natural to work with.

This makes room for a future 'try again' callback to allow ringing someone again after a timeout, and also forced me to look for a simpler solution to the duplicate leave sound effects. I exposed the intent of the ringing attempt to the call UI so I can later use it in the header.

src/state/CallViewModel/CallViewModel.test.ts

@@ -1386,7 +1386,10 @@ describe.each([
           });
 
           // Should ring for 30ms and then time out
-          expectObservable(vm.ringing$).toBe("(ny) 26ms n", yesNo);
+          expectObservable(vm.ringingIntent$).toBe("(ab) 26ms a", {
+            a: null,
+            b: "audio",
+          });
           // Layout should show placeholder media for the participant we're
           // ringing the entire time (even once timed out)
           expectObservable(summarizeLayout$(vm.layout$)).toBe("a", {
@@ -1425,7 +1428,10 @@ describe.each([
           });
 
           // Should ring until Alice joins
-          expectObservable(vm.ringing$).toBe("(ny) 17ms n", yesNo);
+          expectObservable(vm.ringingIntent$).toBe("(ab) 17ms a", {
+            a: null,
+            b: "audio",
+          });
           // Layout should show placeholder media for the participant we're
           // ringing the entire time
           expectObservable(summarizeLayout$(vm.layout$)).toBe("a 20ms b", {