Make login and signup work with new frontend

[Azeem-Khan1]

• Change IDs in Dash's login code to match with FE (30 min)
• Complete error handling / sanitizing (30 min)
• Fix CORS stuff and things (15 min)
• Rework edge case handling (15 min)
• Functional show/hide buttons for password & pswd confirmation (1 hr)

Total time: 2.5 hours