* Use native js-sdk group call support
Now that the js-sdk supports group calls natively, our group call implementation can be simplified a bit. Switching to the js-sdk implementation also brings the react-sdk up to date with recent MSC3401 changes, and adds support for joining calls from multiple devices. (So, the previous logic which sent to-device messages to prevent multi-device sessions is no longer necessary.)
* Fix strings
* Fix strict type errors
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