Creating a custom user model for extensibility

[katporks]

This pull request focuses on future-proofing the user model by extending the default base class. Implementing this enhancement in the future could pose complexity in terms of migrations, especially once the staging and production schema is established. Anticipating the extension now makes migrations less intricate for future modifications or additions of fields.

The default Wagtail User model includes the following fields:

• Username
• Email
• First name
• Last name
• Password

Users can assume one of the following roles:

• Administrator: Full access to manage any object, setting, or user.
• Moderators: Access to creating and publishing drafts (no access to settings).
• Editor: Can create drafts but not publish them (no access to settings).

Feel free to suggest any fields for modification or addition!

Additionally, this pull request enables running pytest in a local setup.

[spwoodcock]

I moved the requirements-dev.txt install to a separate stage two two reasons:

• As the inheritance order changed we don't need to pre-compile the dependencies for debug.
• We also don't want to run the debug image as root. That's only needed for the test (ci) image.

Otherwise looks good to me 👍