Add support for playing a sound when the user exits a call.

[Half-Shot]

Implementation for #343 (comment)

This is the trickiest thing to get right by far. We can't use the view model as we do for the other sounds since the act of hanging up will kill that off first. The next problem is we have many states that can occur from hanging up depending on what mode the application is in:

• For SPA mode, this is a trivial case of adding an audio element to the end call screen.
• For video rooms / group call rooms where we surface the lobby screen, we can just play a sound when we reach that screen.
• For 1:1 calls where the widget closes when we hangup...we have a problem. We need to keep the widget alive long enough to play the sound but without making it hang around and feel unresponsive.

One option here is that for call leaves only, we make this the client's problem. The problem with this is we'd need to provide those audio files up to Element Web / EX etc, which is a nightmare if we decide to change the sound at any point. Supposedly we could make them part of compound, but that's a whole new design thing to consider.

So, this PR does the first two trivial things and currently holds the window open for 1:1s. We could also decide that 1:1s can be different as it's more obvious you've hung up when the widget goes away, as oppose to the SPA/group call use case.

[fkwp]

It seems that the playout device for the join/leave sounds is the OS default device rather than the configured audio device from the settings.

[Half-Shot]

It seems that the playout device for the join/leave sounds is the OS default device rather than the configured audio device from the settings.

#2863 will fix this.