Add skeletal EventStringifier

This class is supposed to generate the user-visible messages from IrcEvents,
i.e. it generates MessageEvents. For now, this lives in the CoreSession, which consumes
and converts the MessageEvents into the usual Messages that are then sent to the client.

In the future, this whole class should move into the client, in order to move string generation
out of the core. There's no need to store plaintext strings in the database if we can as well
simply store the IrcEvents, plus this will allow per-client translations.