[refactor->develop] backend auth migrates from jwt to Django sessions

[Typogalaxy]

• remove djangorestframework-simplejwt package and related logic
• use login() and logout() provided by django.contrib.auth to manage sessions
• add UserSession Model to provide a convenient interface for users to inspect their sessions
• add SessionTrackingMiddleware to add sliding expiration and last_accessed_at feature
• write a custom authentication backend EmailBackend, because email and password is in two different models
• add clear_expired_sessions task, which uses django clearsessions command, to clear expired sessions