Add setting for redacted notifications

[kontaxis]

This is a request for a setting so that notifications don't include anything about the message that triggered them.
Given that notifications travel from the ZNC service to Palaver servers to Apple servers to the client, users may want to obscure the sender, network, and content of a message.

"Settings > Notifications > Show Message Previews" sort of does that but still includes the sender and the name of the channel.

See #70

[beren12]

iOS can do this itself, it'll hide the message contents if the phone is locked if you have that setting on.