build: Moved user and group management commands and unit tests to edx-django-utils

- Removed manage_user and manage_group commands and their unit tests from edx-platform and added then to edx-django-utils.
- Modified User.post_save signal to ensure the user profile is created when manage_user management command is run to create a user.
- Added edx-django-utils to INSTALLED_APPS for LMS and Studio.
- Moved generate_password from openedx.core.djangoapps.user_authn.utils to edx_django_utils.user along with its unit test.