Add login and authentication functionality

[stark-cybertech]

Meet@Mensa users need to be able to login in order to add and or update their preferences and interests, input the times they will be having lunch in order to be matched, and access live features like chat and the AI conversation starter.

The goal of this issue is to implement login functionality and access to accounts securely, while limiting the amount of sensitive data that is stored by Meet@Mensa.

Subtasks

• Decide rather to use a custom login system, or rely on TUM SSO
• ...

DoD:

• Basic login functionality is implemented.
• Passwords are stored securely or their use is avoided entirely by relying on TUM SSO
• User accounts offer persistence of preferences and other data.