Improve User model

[trungams]

Description

We now offer password authentication for user who don't have a WCA account. There is still an option to connect with WCA after you signed up.

Registration can now be done on our website.

This merge is necessary to resolve any conflict we have before developing event scheduling any further.

Motivation and Context

see above

How Has This Been Tested?

Only tested manually. More automated tests to come when we agree on this design

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.