Hotfix/user serializer expansion

[MaxWasUnavailable]

Description

Please include a summary of the change and which issue is resolved. List any dependencies that are required for this change.

Created NestedUserExtraInfoSerializer, which is added as a nested serializer to the User serializer; adding its info the User object when requested through the User serializer.

We now also create the linked UserExtraInfo object automatically when a user registers via the RegisterSerializer or is created through the regular UserSerializer.

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce.

• Created new users through API & Register routes.
• Checked UserExtraInfo in admin view.

Checklist:

Please check [x] all that apply.

• [x] My code follows similar styling to existing code in this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, at the very least in hard-to-understand areas
• [ ] I have made corresponding changes to documentation / READMEs
• [x] My changes generate no new warnings
• [x] I have remade my frontend and backend Docker containers to ensure my changes do not break the setup for these
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [x] I have checked my code and corrected any misspellings

[MaxWasUnavailable]

@pascalstockert Should be fixed 👌🏻