Make Network.Nick optional, default to the user's username. This
will allow adding a global setting to set the nickname in the
future, just like we have for the real name.
References: https://todo.sr.ht/~emersion/soju/110
This is a mecanical change, which just lifts up the context.TODO()
calls from inside the DB implementations to the callers.
Future work involves properly wiring up the contexts when it makes
sense.
