Switch accounts app to django-nano-account

This MR remove the accounts app and replaces it with django-nano-account which is a refactored and re-usable version.

https://gitlab.bham.ac.uk/owendz-protein-databank/django-nano-account/-/tree/initial-refactor?ref_type=heads

As per smlm I've chosen to include django-nano-account as a git submodule and the pip install it through requirements.txt. The main difficulty is how to update the existing database to the new User model. I've tested the following procedure locally and seems to work:

1. Open database: sqlite3 db.sqlite3
2. Delete django_migrations table: DELETE FROM django_migrations; .quit
3. Fake migrations: python manage.py migrate --fake