This adds our short device ID to the basic auth realm. This has at least
two consequences:
- It is different from what's presented by another device on the same
address (e.g., if I use SSH forwards to different dives on the same
local address), preventing credentials for one from being sent to
another.
- It is different from what we did previously, meaning we avoid cached
credentials from old versions interfering with the new login flow.
I don't *think* there should be things that depend on our precise realm
string, so this shouldn't break any existing setups...
Sneakily this also changes the session cookie and CSRF name, because I
think `id.Short().String()` is nicer than `id.String()[:5]` and the
short ID is two characters longer. That's also not a problem...
439c6c5b7c