✨(models) make user and authentication work with Keycloak and admin

The admin was broken as we did not worry about it up to now. On the frontend
we want to use OIDC authentication only but for the admin, it is better if
the default authentication works as well. To allow this, we propose to add
an "email" field to the user model and make it the identifier in place of
the usual username. Some changes are necessary to make the "createsuperuser"
management command work.

We also had to fix the "oidc_user_getter" method to make it work with Keycloak.
Some tests were added to secure that everything works as expected.

Makefile

@@ -177,7 +177,7 @@ migrate:  ## run django migrations for the people project.
 
 superuser: ## Create an admin superuser with password "admin"
 	@echo "$(BOLD)Creating a Django superuser$(RESET)"
-	@$(MANAGE) createsuperuser --username admin --email admin@example.com --no-input
+	@$(MANAGE) createsuperuser --email admin@example.com --password admin
 .PHONY: superuser
 
 back-i18n-compile: ## compile the gettext files