Use a custom user model by default

[wsot]

🎯Aim

Apply the good practice of using a custom user model from the beginning.

📕Context

Good Django practice is to use a custom user model from the very beginning of a project. The template currently doesn't do that, which means any user of the template has to do so.

📝Relevant resources/doc's/people

Random links:

• Learn Django: Django Custom User Model
• The power of customizing the Django user, why you should start early

✅Acceptance Criteria

• A custom user model exists (in an accounts app maybe?) and is set up by default.
• Documentation and tests for the custom user model exist

[a-musing-moose]

Completed in #47 and #48