tweak(mumble/server): security improvements

[Korioz]

This PR features a bunch of improvements addressing mumble server abuses (unwanted users connecting from the official mumble client trolling legit users).

• Admin password can now be provided without being connected this allows us to disconnect no-admin users not responding to some criteria.
• Disconnect users connecting from the official mumble client without an admin password set.
• Disconnect admins who are providing a server id in username field, this help prevent unwanted behaviour with natives or future implementations.
• Disconnect non-admin users who are providing no server id or a server id already used in username field.

[blattersturm]

This.. still doesn't prevent much. Intended approach is to have some sort of shared secret-based token exchange support.

Half-assed changes that will instantly be circumvented don't help anyone, sadly.

[thorium-cfx]

Closing this as of blatterstrum's reply, issue has been backlogged to address it later.