Summary: This is the implementation of the new login flow as designed in figma. It also deprecates all of the old login code and significantly simplifies the login.

Test Plan: Tested on login with CLI/UI, etc. Still needs more testing around signup.

Reviewers: michelle, philkuz, nserrino, #engineering

Reviewed By: michelle, #engineering

JIRA Issues: PC-444

Differential Revision: https://phab.corp.pixielabs.ai/D5552

GitOrigin-RevId: 104444d0a4b32c4c711c0e91f9eb8ed36ecf8f65