Make sure users can't complete sign up if they don't accept the T&Cs

[alexwlchan]

There's a TODO comment here that suggests we might not be doing that:

identity/packages/apps/auth0-actions/src/redirect_to_full_registration.ts

Line 12 in fce8df9

// TODO: It would be good to improve this further by checking for app_metadata.terms_and_conditions_accepted here

I want to double check it and update that TODO.

[alexwlchan]

Testing this, it looks like we have client-side validation that you can't submit the form. I want to double-check that's validated on the server-side (and maybe recorded in Sierra?).