Add registration functionality to OAuth2 authentication

[shinyay]

Related to #2

Implement user authentication system using OAuth2 with GitHub, including login and registration functionalities.

• app.py:
  • Add registration route and logic for OAuth2 flow.
  • Update login route to handle both login and registration.
  • Modify authorize function to include registration logic.
• models.py:
  • Remove password_hash field from User model.
  • Update User model to store OAuth2 user details.
• README.md:
  • Update documentation to include registration functionalities.
  • Modify OAuth2 setup process to include registration steps.
• templates/register.html:
  • Create registration page for OAuth2.
  • Include form for user registration.
• tests/test_authentication.py:
  • Add unit tests for registration functionalities.
  • Update existing tests to reflect changes in authentication flow.

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