This implements sql.Scanner and sql/driver.Valuer, so that we can
load/store time values into SQLite with the format we want, and
properly handle NULL (which the go-sqlite3 package doesn't do
correctly).
The soju username is immutable. Add a separate nickname setting so
that users can change their nickname for all networks.
References: https://todo.sr.ht/~emersion/soju/110
Some WebPushSubscription entries aren't tried to a network, in
which case the "network" column is NULL. But then all users share
the same row. Oops.
Fortunately network-less subscriptions aren't used for anything
yet, they're just stored. So the impact should be minimal.
