This adds a table of "cancelled group rings", to handle out-of-order
delivery between a cancellation and a ring. (This can happen when the
cancellation comes from a linked device rather than the original
ringer, though it's very unlikely.) Thirty-minute-old cancellations
are cleared lazily.
This also adds a new IncomingCallControls to replace the usual
CallControls in a GroupCallViewController.
And finally, there's a lot of code that was 1:1-call-specific that now
handles group calls as well, either by being more general or by
checking which kind of call we have.
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