lib/api: Add cache busting for basic auth (ref #9208) (#9215)

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...

lib/protocol/deviceid.go

@@ -13,10 +13,15 @@ import (
 	"github.com/syncthing/syncthing/lib/sha256"
 )
 
-const DeviceIDLength = 32
+const (
+	DeviceIDLength      = 32
+	ShortIDStringLength = 7
+)
 
-type DeviceID [DeviceIDLength]byte
-type ShortID uint64
+type (
+	DeviceID [DeviceIDLength]byte
+	ShortID  uint64
+)
 
 var (
 	LocalDeviceID  = repeatedDeviceID(0xff)
@@ -94,7 +99,7 @@ func (s ShortID) String() string {
 	}
 	var bs [8]byte
 	binary.BigEndian.PutUint64(bs[:], uint64(s))
-	return base32.StdEncoding.EncodeToString(bs[:])[:7]
+	return base32.StdEncoding.EncodeToString(bs[:])[:ShortIDStringLength]
 }
 
 func (n *DeviceID) UnmarshalText(bs []byte) error {