Add a rolling key for posting

[rslay]

Add a rolling identification key for posting, so that if the viewing key is compromised on the user's end, nobody can pretend to be them with an old key.
Since the passcode is already secured and only sent over once, this would be the next step to secure the identification method for users

[rslay]

This remains true for version 1.0.0, and a rolling token for posting would be a great way to prevent posting as someone else if their browser session is hijacked.

It might be excessive, but I want to make this chat as secure as possible for all scenarios. If it does not encumber users, it's worth implementing.