Integrate New Paid Sign Up Flow

[TimCliff]

From @nedsteem in the condenser repository:

On signup.steemit.com:

1. Integrate Steem.Ninja credit card - based signup flow (https://steemit.com/introduceyourself/@steem.ninja/steem-ninja-account-sign-ups-with-credit-card-and-referral-reward-system)
2. Deprecate Wait List flow.

Ref: https://github.com/steemit/condenser/issues/3152