Add custom User model to app

[jd7h]

Fix for issue #44. Reason: best practice according to the Django docs:
https://docs.djangoproject.com/en/3.0/topics/auth/customizing/#auth-custom-user

Upon this commit, you need to throw away your migrations and start over with a clean slate.
Instructions:

### Source: https://simpleisbetterthancomplex.com/tutorial/2016/07/26/how-to-reset-migrations.html

# Delete migrations
find . -path "*/migrations/*.py" -not -name "__init__.py" -delete
find . -path "*/migrations/*.pyc"  -delete

# Drop database
# (delete db.sqlite3 or see https://stackoverflow.com/questions/34576004/simple-way-to-reset-django-postgresql-database

# Create migrations and generate DB schema
./manage.py makemigrations
./manage.py migrate