Prevent user from sending replies when client is not authenticated

[heartsucker]

Fixes #243

Testing:

• Start client
• Skip login
• See warning instead of reply box
• Sign in
• See reply box
• Sign out
• See warning again

Also:

• Check that the type annotations I added are correct

[sssoleileraaa]

lgtm w/ one nitpick/ allie's-humble-opinion

• Behavior works as documented in the PR description
• Type annotations are helpful and look correct
• New is_authenticated Client property lgtm, new authentication_state pyqt signal lgtm

[nitpick] Does it make sense to carry the concept of authentication all the way down to the widget level? E.g. This might read more clearly:
_on_authentication_update -> _show_replybox
self.controller.authentication_state.connect(self._on_authentication_update) -> self.controller.authentication_state.connect(self._show_replybox)
To me it's just a little easier to understand what the authentication state affects.