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

• [x] Fixing remaining sign-in issues folks are experiencing
  • https://github.com/GSA/notifications-admin/issues/1500
  • https://github.com/GSA/notifications-admin/issues/1501
  • https://github.com/GSA/notifications-admin/issues/1546
• [x] Re-wiring all of the service invite logic into the Login.gov sign-in flow
  • https://github.com/GSA/notifications-admin/issues/1474
  • https://github.com/GSA/notifications-admin/issues/1468
  • https://github.com/GSA/notifications-admin/issues/1459
• [x] Making sure the E2E test user auth checks are still functional with the Login.gov sign-in flow now in place
  • https://github.com/GSA/notifications-admin/issues/1459
• [x] Re-enable email verification checks as a part of the Login.gov sign-in flow (already done)
  • https://github.com/GSA/notifications-admin/issues/1425
• [ ] Making sure the checks are in place to only allow *.gov email (similar to cloud.gov)
  • https://github.com/GSA/notifications-api/issues/841
  • https://github.com/GSA/notifications-admin/issues/1481
• [ ] Making sure that phone numbers belong to the user - this is for the "complete your profile" page
  • https://github.com/GSA/notifications-admin/issues/1528
• [x] Update our documentation to capture the new setup requirements
  • https://github.com/GSA/notifications-admin/issues/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.

[ccostino]

Quick note that we need to take care of the following, as noted by @heyitsmebev and @terrazoon:

we need another ticket to address the technical debt to refactor or removing the sign-in page and integrating the login logic into the landing page. This will ensure that the login is intact and functioning as before, given its complexity.

I'll get an issue written up for this soon, but didn't want to lose track of it!