Fix: sfu player would try subscribing when sfu disconnected

[Belchy06]

Relevant components:

• Signalling server
• Frontend library
• Frontend UI library
• Matchmaker
• Platform scripts
• SFU

Problem statement:

In Cirrus, when the SFU disconnects, the player representing the SFU would try and subscribe to the SFU. This is just a simple type, the player should instead unsubscribe.

Solution

Fix the type. The player now unsubscribes on SFU disconnect.

Fixes #171

Test Plan and Compatibility

Tested streaming both with and without the SFU. Checked that on SFU disconnect, the player will unsubscribe instead of subscribing.

[lukehb]

lgtm