[Waiting on #4807] Prevent chatting with NPCs while deaf

* Prevent using the 'C'hat command with an NPC while deaf.
* Prevent NPCs from talking to you while you're deaf.
* If an NPC is trying to mug you, your lack of response turns them hostile.

I'm reasonably sure this works, but #4807 needs to be fixed first because, while I can clearly see that the NPC does turn hostile, this instantly crashes the game for reasons utterly unrelated to this PR (I can reproduce it on the unmodified master branch).

Fixes everyone in the game apparently knowing sign language: Fixes #4760.