Add a "null" auth backend

Not built by default, but can be added with `go build -tags nullauth`.
Enabled by running tokidoki with `-auth.url null://`. Very useful for
simpler debugging when you don't want manage test accounts with actual
passwords.

auth/null_stub.go

@@ -0,0 +1,11 @@
+//go:build !nullauth
+
+package auth
+
+import (
+	"errors"
+)
+
+func NewNull() (AuthProvider, error) {
+	return nil, errors.New("NullAuth is disabled")
+}