#6637739 P2: customize signup page

Changes proposed in this Pull Request

• Customizes the signup page for P2 flows, as described in p9lV3a-2Lp-p2. This is part of the P2 Onboarding project.
• Refactor: move parts of the customized styles for accept-invite signup to logged-out-form so it can be reused by the start/p2 flow

Testing instructions

• Regression test: verify that non-P2 signup flows have the regular signup screen.
• Regression test: verify that accepting invites for non-P2 sites have the regular signup screen.
• Check that nothing looks broken for the following:
  • sign up screen when accepting P2 invites
  • sign up screen for the /start/p2 flow
• Check that you can successfully sign up via the /start/p2 flow

Screens

1. Before expanding the email signup form

2. After expanding the email signup form (at 80%)

3. (Refactored so styles for email signup form are shared) The accept-invite-logged-out screen