Summary:
This adds a new landing page for cognito sign-up, intended for
use by new appsumo users.
Their email address is pre-filled and locked down, and sign-up
is by entering a password.
The page is very crude compared to hosted cognito - especially
in error reporting! - but having the address filled in more
than makes up for that.
The flow does not quite connect with the new billing signup.
I think we can do that through the regular "welcome" process,
which will list the user's team site. When the user visits
that site, we could detect that we are on a site with no
domain set yet and for which the user is a billing manager,
and trigger a visit to the appropriate billing page.
Test Plan: manual - hard to test through cognito email step
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2880
36d5e7870e