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...

test/http_test.go

@@ -173,7 +173,7 @@ func TestHTTPPOSTWithoutCSRF(t *testing.T) {
 	}
 	res.Body.Close()
 	hdr := res.Header.Get("Set-Cookie")
-	id := res.Header.Get("X-Syncthing-ID")[:5]
+	id := res.Header.Get("X-Syncthing-ID")[:protocol.ShortIDStringLength]
 	if !strings.Contains(hdr, "CSRF-Token") {
 		t.Error("Missing CSRF-Token in", hdr)
 	}