Add a new flag to disable users. This can be useful to temporarily
deactivate an account without erasing data.
The user goroutine is kept alive for simplicity's sake. Most of the
infrastructure assumes that each user always has a running goroutine.
A disabled user's goroutine is responsible for sending back an error
to downstream connections, and listening for potential events to
re-enable the account.
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.
