Post-rollout cleanup for Login.gov integration

[stvnrlly]

There are several loose ends we need to tie up after the full transition over to Login.gov: user service invites, E2E test authorization checks, and *.gov email address verifications.

Status: Landing

Implementation Sketch and Acceptance Criteria

• Fixing remaining sign-in issues folks are experiencing
  • Investigate authentication issues between Login.gov and Notify.gov #1500
  • Bug: Users are still asked to complete their profile when attempting to accept an invite for someone else #1501
  • Remove "Effective April 16" Login Landing Page #1546
• Re-wiring all of the service invite logic into the Login.gov sign-in flow
  • Add conditional logic for new-user invitations to give more Login.gov information than an existing user #1474
  • When user invite members, user does not get redirected #1468
  • Refactor the remaining pieces of the sign-in process #1459
• Making sure the E2E test user auth checks are still functional with the Login.gov sign-in flow now in place
  • Refactor the remaining pieces of the sign-in process #1459
• Re-enable email verification checks as a part of the Login.gov sign-in flow (already done)
  • Bug: user email validation isn't wired up with Login.gov integration #1425
• Making sure the checks are in place to only allow *.gov email (similar to cloud.gov)
  • Remove email check from Login.gov login flow notifications-api#841
  • Bug: non-.gov emails are allowed to register #1481
• Making sure that phone numbers belong to the user - this is for the "complete your profile" page
  • Confirm that a new user must validate phone number (via MFA code) before it can be used in their profile #1528
• Update our documentation to capture the new setup requirements
  • Document changes in Login.gov setup process in our README and developer documentation #1518

Security Considerations

• We need to preserve the mechanisms that were in place with the old sign-in flow with the new Login.gov sign-in flow to make sure that the system still functions and operates as expected while still properly authenticating users.
• Only email addresses with *.gov are allowed into the system.