Use new nick for finding hash of Nick messages

Use new nick for finding hash of Nick messages

For nick change messages, use the new nickname for calculating the
sender hash, not the old one.  This more closely matches
expectations.

Works around a limitation in the Quassel theming engine that doesn't
allow different styling for multiple nicks in one message.
Unfortunately, rewriting the necessary parts of the theme engine to
support multiple styles per nick is above my understanding right now.