Show warning message when joining call from multiple devices about element-call HOT 3 CLOSED

We decided that we would initially not support multiple devices, but need to show a warning message to the user that they are calling in from multiple devices. Supporting calls from multiple devices will be covered in this ticket #47

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I have some questions on the shape of the m.sources array that need to be answered before I can look into this further. Also, what should happen if you call in from the same device? And what happens if you have left the call but did not update the m.sources array to remove your device?

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I added hasLocalParticipant to GroupCall and useGroupCall which can be used to check if you have an existing session for the group call. We now display a warning in the room setup screen when this is the case.

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