luci-base: properly handle authentication without authenticator

Some controller actions like the ones in "servicectl" require authentication
but are not meant to provide an authenticator because they're only invoked
by scripts.

Rework the dispatcher logic to handle this situation and only bail out if
an authenticator name other than "htmlauth" is set.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>