Support 2FA

[ShadowKyogre]

So, here's my initial draft of integrating 2FA support into PageProtector. So far it works. Embeds form when appropriate, doesn't render page if the 2FA couldn't start, etc. But I'm still a bit confused at how to check if a 2FA flow is active without loading the entire Tfa class instance before the user signs in (regardless if anonymous or not). I haven't figured out a way around that, which is why there's still usage of this snippet for the second of the 2FA phase.

$tfa = $this->wire('modules')->get('ProcessLogin')->getTfa();